We've had alot of dust here in the past. I've started using a pre-filter to the outer-filter.
something like
I've found it works really well for me. I had a NASTY lookin' pre-filter, but the outer filter was NICE and clean, much less the inner filter. And they really breathe very nicely. It's not necessary, but the 3rd filter element is nice insurance for me.
Under that I run a Kyosho air filter assembly. They protect the outside edge of the inner filter VERY well. And the way the pre-filter fits, it seems to compress the edge of the outer-filter against the casing pretty nicely.
I also use 2-3 different filter assemblies throughout 1 full race day. Maybe just 1 on the best of days, but not common.
Then I wash @ home. If I see any specs of dirt on the inner-filter I chunk it. If I see any dots from the inside of the outer filter, I toss it.
I use the same filter covers on my air filters. I don't have an issue cleaning and re-using filters since I started using the filter covers. Filters look really good after a race day, and I now wash and re-use filters.