I'm using the pelican brand PSP screen protector.
It's $4.50 at best buy, and comes with a lint free cleaning cloth and cardboard squeegee. I've been using Write Right protectors for my pocketPC, and I have to say that the Pelican is MUCH nicer. The plastic is harder and smoother, so the stylus glides much nicer. Also, instead of a sticky adhesive, it seems to have a thin gel coating. That puppy went on there slicker'n snot, and left NO bubbles and no dirt flecks "tenting" up the protector.
The protector height is a perfect fit, and you just cut 5mm exactly off the width to make it fit.

I really couldn't be happier, it works great.