In the past, I have used diatomaceous earth and put it in their dust bath, but have also used sevin dust when I had mites. For severe cases, I have also heard of people doing the using sevin dust in a garbage bag. They put the chicken in the bag with its head sticking out the top, of course. Then, shake the bag, thereby dusting the chicken's body with the dust. If you use either sevin or diatomaceous earth, make sure you wear a mask as it can be dangerous to breathe in.
I recently read on this site that people use wood ash, which would be much safer and less expensive. I'm going to try it this year instead of going the pesticide route! Good luck!