Adding a Turntable Mat

   I have an older turntable (Pioneer PL-530). Am the original owner. Have done a few things to keep this table operating as original as possible. Yeah, I know this isn't a high end table by any means. Rather than receiving a flood of advice on what tables are better, I would like to hear from someone who may have knowledge/experience with different types of table mats.
   My table has no VTA adjustment capability. The stock platter mat is rubber. Seeking minor/inexpensive tweaks and if a different mat may or may not provide any perceptible improvement. If it's inexpensive and provides no problem.
   Here's my question.....For example...If I installed a thin cork mat, could that mat simply be placed on top of the existing rubber mat or should the rubber mat be removed first and then just use the cork mat? Remember, this table doesn't have VTA adjustment.

Turntables of this era came with a ribbed rubber mat that does not support the record over the entire playing area. Ken Kessler of Hi-Fi News has been experimenting with mats and weights that fit over the spindle, in recent issues. These are all relatively inexpensive, so much so that one can try and sell on if need be. I had a Pioneer PL-12D and started by inverting the rubber mat placing it ribbed side down. Cork or felt mats are worth a try. I replaced the headshell on mine, which was better than the standard issue. A new drive belt might also be appropriate.
Don't worry about the VTA. It changes anyway with different cartridges.
You want to remove the rubber mat and adhere the cork mat to the plater with a spray contact cement. The mat should have a depression for the record label. Lacquer thinner can always remove the contact cement. 
Lots of good advice here!

I second the motion to lubricate or make sure the bearing on your TT is properly lubricated! A good habit to get into is to check that on an annual basis or more frequently, depending upon how much you spin your records.

With regard to TT mats, if you haven't done so already, I would recommend some reading on Analog Planet and/or Stereophile, among other reputable publications. If memory serves correctly, I believe Michael Fremer may have done an article or two on this and I don't think he's fond of felt mats. As I recall, I think he said they generate static electricity and would wreak havoc on a stylus in an oops moment.

Before experimenting with mats, I would recommend using a record clamp if you aren't already. I very much like and have been using this one for quite some time now:

Michell - Reflex Record Clamp | Shop Music Direct

Check the spindle clearance on your Pioneer to make sure this will work or just ask Michell, Music Direct or whomever.  There are other less expensive clamps like The Record Doctor and a few others. I've never used anything other than the Michell and found I don't need to use the included optional washer recommended for significantly warped records. I only have 1 record with a slight but significant warp. It's unplayable without the Michell clamp but plays beautifully with it and no washer. Record pucks or weights accomplish the same idea. However, I've never liked the notion of putting significant extra weight on a TT bearing.

If you decide to do the record clamp or weight thing, just for fun, play your favorite record without the clamp or weight, first. Then, play it again with the clamp or weight right afterwards. I wasn't expecting much when I did this. However, I was quite pleasantly surprised with the results.

Good Luck!  And please let us know how you make out! 
It’s very funny that the OP asks about mats, and he gets advice about a half a dozen other different tweaks by and large, with almost no mention of turntable mats. I must disagree with anyone who maintains that mats make very little difference. Rather, I think of a turntable mat as vitally important to the sonic character offered by a turntable, tonearm, and cartridge. It is rather like seasoning your food. Judicious choices can significantly affect your dining and listening pleasure.Problem is that I think each individual has to decide for himself what mat sounds best on his turntable in his system. So I would not give specific advice, except perhaps to get rid of those floppy heavy rubber mats that typically were delivered with vintage Japanese turntables of all types. Start from there. My additional advice would be to get rid of the OEM rubber mat entirely; do not place an aftermarket mat on top of the OEM mat.
Seeking minor/inexpensive tweaks and if a different mat may or may not provide any perceptible improvement.

lewm, that is a two part question.  

Part a) "seeking minor/inexpensive tweaks" and   

Part b) "if a different mat may or may not provide any perceptible improvement."   

The mat question was answered in the first three posts. That leaves "minor/inexpensive tweaks" which can go on forever.