Motor Saver or another dual foam filter. The outer filter blocks most of the dust, and the inner oiled filter locks out the rest. It doesn't need as much cleaning as the stock filter, since it has the dry foam protecting the oiled foam.
I've got a motor saver and it works great. It's easy to clean and it works good, and the replacement filters are really cheap to buy online, on ebay, or at your local hobby shop.