Ive had my DS for a year now, but unfortunatly due to the lack of games on the market until just now I havent really had a chance to consider a screen protector. but now that animal crossing DS is out...
on a more serious note, I use a static cling screen protecter I paid $5 for my PSP. Best $5 I ever spent in my life.If it gets dirty, i just pull it off and wipe it with a cloth and it clings back on to the PSP screen.