if you ride in mud and slop get a foam filter, sandy conditions get a k&n
actually the K&N will let more sand through than the UNI style foam filters. There have been guys who find sand in the intake tract with a K&N, switch to UNI and no more problems
if you've got the coin........i'd get one of these
Yamaha Blaster CFM Aluminum Air Box Airbox Intake: eBay Motors (item 350367939645 end time Jul-15-10 16:55:13 PDT)
uni is my best bet then its mostly dirt trails were ill b riding