As stated before make sure the filter is properly oiled (filter  oil not motor oil)  Also, I always like to apply a film or wheel bearing (or any type) of grease to the bottom lip of the air filter.  by bottom lip, I mean the 3/4 inch wide part that makes contact with the air box.  The factory filter set ups are not the best and dont clamp on, so the grease is an added measure of insurance to keep stuff from going under and around the filter.