If its a paper element filter you just blow it out with compressed air, if its a sponge type filter wash it in some petrol and let it dry off, then thin a bit of 2 stroke or engine oil down with petrol and dip the filter in it squeeze it a little bit to get rid of the exess and leave to dry.
This will condition the filter after cleaning and the oil residue is better at trapping the dust in the air stopping it getting into the engine than just the dry sponge alone.