If your P2 has the MDF platter, I would replace that with the glass platter first.
After that, depends on how much you want to spend. For about $20, you can get something along the lines of the ExtremePhono None Felt. The original is discontinued, but plenty of people still sell them because it's basically just a piece of non-slip carpet backing. You can cut one yourself but you'll never get it perfectly round. Look for a seller named yoder365 on ebay. He does a great job. It's cheap and very effective.
For around $70, the Herbie's Way mats are a big step up and I wouldn't spend much more than that on a mat for the P2. I've tried the cork and sorbogel mats with much less success, and they cost more.
I've had various Planar 2 and 3 models over the years and here are the most dramatic improvements you can make to them, in order:
1. Replace the plastic subplatter with a machined metal one. The best I've found is the Groovetracer. It can take a P2's performance to P3 levels and beyond in some ways.
2. Glass platter. Or acrylic, depending on your tastes. Both are a big step up from MDF. I like glass.
3. Replace the arm's stock counterweight with something from Groovetracer or Mitchell.
