Thoughts on extending speaker life.

For those of us who own speakers that are past a few years old, and those same speakers utilize dynamic drivers, what are your thoughts on extending the life of the rubber surround ( assuming that your driver utilizes this kind of design).
In general, as the driver ages, the likelihood of the rubber surround failing increases. Without actually replacing the driver ( not possible in some instances) what do you suggest to extend the life of the driver(s), if anything?

For those with other types of driver design, what are your concerns about the extended usage of this kind of driver as it ages?

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I agree with Elizabeth. Though I can't comment on the advisability of applying silicone to the surrounds, it is a bad idea to apply petroleum-based products unless you can ascertain the exact composition of the surround - not all "rubber" surrounds are created equal. 

Some like to rotate their woofers occasionally, to counteract potential suspension sag. From everything I've read and encountered, it takes many years for a good suspension to suffer that phenomenon. 

One of the best things you can do to prolong woofer life is to keep the grills installed, at least when not listening. Many speakers are voiced for use with the grills intact anyhow. In fact, 4 of my 6 pairs are voiced that way and sound better with the grills in place.
You have great speakers, forget about  it.

Listen to music, life's too short to be concerned. 

Perhaps in 20years, when the drivers show their age, you can then seek out a complete set of NOS drivers.

Oh but wait, those too, have been sitting around...

I have a pair of KEF Q10s bought new in '94 or something...I originally used them as near field monitors in my studio...these things are maybe 25 years old and still sound fabulous with the rubber surrounds pliable and seemingly fine, showing no signs of age other than the minor scratches and dents here and there, and now I stick them in the windows to my deck for outdoor speakers when needed (driven by a designated mosfet power amp). Great speakers.
bought my VonSchweikert db99s in 2005 and the surrounds show no deterioration. I never "treated" the surfaces of the speaker surrounds in any way. The sound is still stellar to me. I'm very happy I spent the money since now the company’s products are priced in the stratosphere. I guess that’s where their market is.
@daveyf --

For those with other types of driver design, what are your concerns about the extended usage of this kind of driver as it ages?

I’m not concerned, really, but I’m invested in this subject nonetheless. My speakers are all-horn loaded using compression drivers for the midrange and tweeter, and a (hidden) 15" bass driver with treated accordion cloth surround in a folded horn. None of these drivers are directly visible (only the exit area of the compression drivers via fine-masked grilles), and so their diaphragms and surrounds aren’t exposed to light - not even the bass driver, which is situated behind a some 3x13" slot (its exit area into the folded horn). Treated cloth surrounds at the end of paper cones, even when exposed to light, usually last decades, and the Mylar and paper composite material of the tweeter and midrange drivers respectively should have close to indefinite lifespan. All things considered (including cross-over components, cabinetry, etc.) I would expect my speakers to perform immaculately (way) past 30 years, quite comfortably.

If anything it seems to me regular use of one’s speakers is the best way to actually preserve them, and in your specific case I wouldn’t worry about the rubber surround of the of bass/midrange driver. Try to avoid them being exposed to direct sunlight, and with rather normal atmospheric conditions, humidity and no severe temperature variations (other than what normally occurs when opening windows and such), I gather you should have decades of trouble-free service from your beautifully crafted, well-engineered and high-quality speakers.
An occasional application of Armor All is beneficial for surrounds.
I would never apply anything to my speaker surrounds. It can do more damage than good. As long as you keep your speakers from exposure to any of the known things that can cause degradation of the surrounds, the surrounds should last a lifetime. The things to avoid are UV (from sunlight and fluorescent lighting), Ozone (laser printers are big culprits), VOCs (from paint, household cleaners, furniture polish, metal cleaners and of course cigarette smoke) and excessive heat and humidity.

I purchased my KEF Calinda speakers in 1979 and they are still going strong in my third system. The surrounds are rubber and still look like the day I purchased them after 40 years. My KEF 104/2 speakers were purchased in 1991. They are the biwire version and have foam surrounds and foam donuts on the woofers. After 28 years they still look like new and show no signs of degradation.
Don’t forget about the Most important part of the speakers,
the Heart ,the Crossover , Parts Quality is average at best 
on 90% of all crossovers, upgrade withsame values just much 
higher quality  parts ,including , wiring, and getrid of the cheap gold plated brass that most speakers connectors have as well as your electronics rca and Loudspeaker terminals ,that alone a very noticable sonic improvement.
AR 18S surrounds crumbled after maybe 10 years. Mission 761i will be 25 (years) in a month or so. surrounds look like new. Third pair, 14 years, looks like new, too. Fourth is still too young to tell. None of them has ever been treated or paid any attention to.

Could it be that something about materials used has improved over the last 20-30 years?

I would not apply anything on surrounds. There is more chance I would damage them, poke the hole or something else disastrous, than that I would extend their lifespan by a few more weeks. I do not think it is worth it.
You should be more concerned about components, as the odds are that at least one of them will go first. Silver lining in that is sound quality in general for the dollar has gotten way better in past 5 years imo across the range of gear. Spend your sleepless nights dreaming of a DAC crash and upgrade.

If an older speaker died, count your blessings and move up! 
i bought mine with a living will
when the time comes
i just have to let them go

The only problem I ever had was evaporation of the ferrofluid in tweeters.  This can be serviced with some tweeters, but mine were not in that class.  A sorry problem for a supurlative and expensive speaker, albeit one of a little age.  There are some do-it-yourself recipes for a fix, but only a temporary fix.  So, I got some new speakers.
Early foam surrounds broke down, but have lasted more than 10 years in my experience. Many lasted 20 years or more. Butyl surrounds don’t have the same break down issue. Accordion paper edges last a long time as well. I have customers with speakers I built in the 80’s with paper edge or butyl rubber surrounds that show no signs of wear. I’m using a set of Eminence woofers from 1983 in my bedroom system. I keep checking for wear but they are still soft an pliant. And sound great. Never treated them. The newer foam compounds being used should last many years. You may not have any need to worry. What does your speaker manufacturer say about life span?
Just had my 1979 JBL woofers re-foamed for $180. They sound good. I've had this happen once before. So this is not a big concern. 

Nothing will help foam surrounds. Keep the speakers out of direct sunlight. Meguire's #40 is a rubber treatment with a UV absorbent in it. You can treat butyl rubber surrounds once a year. 
Drivers do not need to be replaced because the surround goes.  There are places to send them for replacement if you don't want to attempt that yourself using available kits.  I had my doubts about how such repaired speaker would sound, but, I've heard a few and they sound good.  There are people that out there that can do all sorts of repairs, including re-coning, and recharging the magnets on drivers.

I am sort of luck in that the types of drivers I have are inherently long lasting (pleated fabric surrounds for woofers).  The midrange drivers are compression drivesr made in 1939 that are still going strong.  
for rubber surround i use talc powder application every 2-3 years and so far they behave like new. Like giving spinach to popeye. Do not touch rubber surrounds with your fingers. I would avoid petroleum based products, they don’t smell nice. My Dunlavys sound and perform the same after so many years.
Get a great speaker like MAGICO!!
I would not hesitate to use armor all if i thought the surrounds needed it.  i have some original polk monitors that are over 30 years old and show no signs of deterioration- but they are rubber surrounds and the grilles have always been in place.   foam surrounds will just need to be replaced if necessary.  not that difficult.   
don't sweat it.   
Age can deteriorate any part, cone, surround, you could burn a voice coil, ....

I have speakers with drivers made in 1956. The tweeter horns and mid-range horns appear to be bulletproof, 63 years old, still sound terrific. I do have spare drivers waiting (as someone else said, they were made in 1956 also, they just haven't done any work).

The 15" woofers (paper with flexible pleated cloth surrounds), this one

I had them re-coned professionally many years ago, have a spare waiting, and re-coned one myself not long ago.

Videos show how to do it, cost is minimal, the hardest part is scraping the old glue off the spider ring.

Based on the success, I re-coned my Velodyne 12" sub, it went well also.

I used to think about 'sag', especially such advanced years, but there is no evidence in mine. The voice coil would rub, you would hear it for sure.

I should have said, 

15" I replaced the coil, cone, surround

12" I just replaced the surround, the cone and coil were fine, and getting the voice coil centered was easy after watching the video.
I am and will continue to use my 1975 Larger Advent speakers connected to a mid seventies  Crown DC300A power amp and a Kenwood C1 Preamp as my living room - main speakers. I have had them re-coned (new foam replacements and old remaining components) three times. Haven't checked, but will most likely have to have them re-coned again. Absolutely worth it.
To keep my 80 -50 year old speakers working well I play them and keep them dry and out of the sun 
@johnk --

To keep my 80 -50 year old speakers working well I play them and keep them dry and out of the sun

Wouldn't you say regular (certainly weekly) use of one's speakers, apart from other named factors, is a vital part in keeping them "up to speed" over time? It seems to me an overlooked aspect in the discussion of how to favorably maintain the performance of speakers through many years. 
"An occasional application of Armor All is beneficial for surrounds."

No offense but a bad advice. It'll make them crack immaturely. 
I use quality speaker covers.  

I would hope all the car guys know the last thing you want to put on rubber is silicone, as it hastens degradation. Or if not then say hello to Griot's Garage.  But if you are worried about ozone well then guess you better do the insides, as electricity produces ozone so there's likely gonna be a lot more of it inside than outside. Not that its worth worrying about even then. But if you are gonna worry, at least get it right.

I have a pair of mid-80s vintage Vandersteen 4s stored in a closet since 1991 (no use since then). I just assumed the surrounds of midrange, woofer, & subs would be trashed. But I also don't know whether Vandersteen used butyl rubber surrounds or something else. Kinda doubt he used foam, but anything is possible. All to say I have some "skin in this game" and don't know what I'd find if I pulled out the 4s and fired them up...

The best thing I got out of this discussion is " suspension sag." Never heard that term. Now I'm thinking about using a speaker bra...
Will speakers last longer if they're used only occasionally?  I have so many and I don't use some of them much at all. Is this a way of making them last? In my case I'm thinking more of parts like crossover caps and such.
LOL! Just listen to the music already, geeez. Speakers last decades so who cares!!!! Rotating woofers, silicone hahaha..
@tlinkie --

Will speakers last longer if they're used only occasionally? I have so many and I don't use some of them much at all. Is this a way of making them last? In my case I'm thinking more of parts like crossover caps and such. 

On the contrary; using them regularly I believe is beneficial to overall speaker performance and -longevity, as I suggested earlier in this thread. Cross-over parts don't degrade with use, but because of time (although it seems quality components can last decades and still maintain specs) - that is, unless they're glowing hot from each playback-workout. Poster @johnk summarized the matter succinctly above. 
Thanks phusis. Some of my speakers are very fine and I do want to maintain them in as good a shape as possible. Can't listen to them all though. I guess this means I have too many. Or not....
too many.

I have saved 'great' stuff for so long, moving it out now bit by bit

physically don't want to pick up anything heavy (bad back, bad knees, bad attitude).

take up too much space

price drops to nothing for most, better to sell when there is more to get out of them.

we are more likely to move on to something 'new to us' than go back.

you know what you will NEVER part with, off with the rest!
Better to have one great set of speakers then several sets of not so great ones. When you can afford better one sell the old ones and move on. Same for all other Hi Fi equipment accept maybe cartridges. Having a bunch to use is great fun.
funny,  no one talked about this   but i do turn drivers180 degrees and tighten the bolts . its b/c of the  big magnet  Russ G  gravity pulling on it   if you look on that back of the driver  you will see a big old magnet on the back of the driver  You won't need Duct tape     
IF the suspension 'sagged', the coil that drives the cone would rub in the magnetic gap, and you would hear it. Never experienced it, never heard of anyone ever experiencing it.

I had a pro re-cone one of my 15" woofer off center. Got it home, mounted, play, what is that?

I had a staple holding interior insulation vibrate loose, fall onto the back of the 15" cone when they were down firing, a weird random sound, you would not think would come from the woofer, what the hell is that. A Sherlock Holmes investigation, I felt so lucky when I found the little bugger hiding under a spider support. 

foam dissolving (resultant permanent sag-off centering), paper cones drying out, cones developing holes, losing acoustic seal, these happen.

If you can get to them, checking, tightening your drivers, especially big ones, is smart  Rotating at that time, seems innocent enough, but, there is potential to compromise the seal at the gasket, so be aware, verify ok, or put back as it was.My 15" woofers, and bigger midrange horns can get loose. Originally 4 clamps/screws, I added a few extra when making new enclosures.

Verify woofer seal for your confidence the problem does not exist: turn your ac system fan off, close the windows and doors, take the grill off, Mickey Hart, volume up, light a match (we used to use cigarette smoke), hold the match as you circle the perimeter, oh happy or oh sad day. Ports: stuff them for the test. While stuffed, listen a while, hear the difference, then open em up, listen again.