I hold upside down and dust chicken with diatomaceous earth (DE), focusing on areas under the wings and around the vent and gently but thoroughly rubbing it into their feathers to get it against the skin where the mites are (why holding chicken head downward is helpful because of direction of feathers). Nontoxic (though you'll want to avoid inhaling it directly or getting it in your eyes) and whatever some nay-sayers say, it definitely works, though you are best repeating this treatment a couple of times if it's a severe case. Or put a box of it in the coop and let the chickens do it themselves. The pillowcase idea would work great too and be less messy--I never thought of that. Food grade DE (what you want) is pretty cheap, readily available, safe, useful for other aplications as well. A big bag is handy to have around and will last you a long time.