It depends on the bag. The are made of different materials, so a blanket method isn't possible. I never washed mine (felt, at first, replaced by whatever material AFF uses). I would take mine outside, and hang them on something. Then beat the snot out of them with a 4' long 1" dowel. Then I'd shake the dust out that dislodged, turn them inside out and vacuum the surface with my shop vac. For the record, AFF doesn't recommend this. They say the dust cake on the inner surface is actually part of the filtration system. Their bags are singed on the inside (burn the fuzzies off) and so the dust only builds to a predetermined level, at which point it falls off; therefore no cleaning is needed. That, however, didn't stop me from cleaning mine once a year or so (when the weather allowed me to take them outside).