I treat mine with permethrin dust (just don't breathe it in and try not to get it in your chickens face too much.) You can get permethrin dips and sprays, too. It is fairly widely available at most feed stores and hardware stores and not too expensive. (Labeled "Garden Pet and Livestock Dust.") You can use an Ivermectin pour-on, or Elector PSP (Spinosad), too, but I think permethrin is probably the most widely available and economical.
I would treat them once a week. Clean up all feathers (if it's lice, that's where they lay the eggs, at the base of the feathers. You can try to remove these, too, for good measure.) I would treat them for at least for one month and see if you have any left, then follow up once a month to make sure they haven't come back.
