I used a cover over the main air filter, and also a fine wire/gauze 'sandstop'. Basicially you have to cut out the air intake 'hood', use silicone sealant to stick the gauze down. Stops larger sand and grime.
I only cleaned my filter once a month, and the 'sandstop' a rub down every couple of weeks - with no apparant (!) problems.
Check out your local Motocross shop.
KTM LC4 640 Adventure-R 2001
KTM Duke II 2000
Kawazaki ZX-6R 1998
BMW F650 1996