Maintenance for Tube Preamps

I've enjoyed my Conrad Johnson tube preamp for a decade or so. Other than replacing the tubes a few times over the years there hasn't been anything else done to it. Are there other maintenance tasks that I should attend to? I believe all the capacitors are polypropylene (no pesky electrolytics!). All seems well, but I want to make sure I'm treating it properly.
"If it ain't broke don't fix it." I own and have owned many pieces of vintage equipment 40+ years old that I start on a variac and they sound terrific without ever having any maintenance besides tubes.
If you’re not blowing fuses and all seems well, all probably is well. But if you’re itching to do something, I know people who swear by the improvement they realize by regularly cleaning and polishing contacts (PC, IC and tube). FYI, cj offers a “renew" service for their older models, but unless you feel that something’s wrong, I’d wait another ten years before I explored that option. I’ve always been happy with cj’s advise when I’ve called.
Other than periodically replacing the tubes (and they generally last longer than most people think), I would just occasionally treat all of the contacts with Caig Deoxit or Cramolin. You don't need much of it at all, but I think it's a good idea. For a quick treatment, just spray a tiny amount of deoxit into the end of your interconnects and then plug it in and rotate the end to spread the love.

I'd also advise testing your tubes before replacing them. Don't replace based upon some arbitrary limit. If something starts to sound wrong or you blow a fuse, test all the tubes. You can probably find an old Eico dynamic tube tester on eBay for about $200 or less.

The only other thing I could think of is just opening it up and making sure to keep it clean/dust free inside.
Generally speaking, if something is wrong you will most likely be able to hear it. The problem with proactive maintenance (especially with vintage gear) is that you can change the tonal quality - and not necessarily for the better.

Contact cleaning is fine. I would advise against changing/replacing any internal parts other than tubes if the preamp is working fine.
Cleaning connections on a regular basis is a good practice, but I advise against products that leave a film because over time it can get messy and cause problems. A good electrical contact cleaner that leaves no film is your best bet.
Rrog and sibelius offer good avdice. Cramolin, is great at first, then leaves black goo...not good. Clean and connect. Change tubes when noise is an issue. Jallen
Thanks to all who responded - good advice! A bit of contact cleaning is probably a worthwhile thing to do.
I would stay away from Cramolin. It does tend to keep cleaning even when you don't want it to and can leave crud behind. I use pro gold but again if it isn't broke don't fix it. Changes can be made sonically to the preamp by replacing coupling caps etc.
By the way, I mentioned Cramolin but haven't used it for at least a decade. Cramolin violates some EPA regulations and was not rated as safe for plastics. Cramolin is no longer readily available, and I only mentioned it in case you happen to have some lying around. The Caig Deoxit Gold 5% solution/Pro Gold are much better. Interestingly, you cannot legally ship Deoxit/Pro Gold through the air - it must travel by ground. And if you order more than one can, it must travel in a separate package. I think this only applies to the aerosol version.

Anyway, the point is if you already have a can of Cramolin lying around, go ahead and use it, but sparingly, wiping off any excess with a lint-free cloth. Since you probably don't have it, order a can of Deoxit Gold 5%. You can find it any number of places online if it's not carried in your area. Musician's Friend had a good deal on it recently, and you can definitely find it on Amazon.
I have used a CJ premier 14 for fourteen years with no maintenance other than tube replacement every two to four years. The only contact cleaning is with a rough cloth or br turning the RCA connectors a few turns. Sounds as good as new. Ed in the service dept can be very helpful. Thet are this week, but try them in the new year.