If they are getting reinfested it is most likely because they are not being killed in the coop or run. You must remove all litter and treat the entire coop with permethrin or Sevin when you treat the birds, and this must be repeated at least once. The recommended treatment is Sevin, though permethrin, some flea medicines and Ivomec Eprinex also work; DE may reduce their numbers but is not effective against an infestation; people have found infestations who were using DE regularly. They are not always on the birds, they will hide in every crack and crevice in the coop, and of course the litter. Except for Ivomec Eprinex, any treatment must be repeated (at least) twice because Sevin, etc. will kill the mites but not the eggs, which will hatch in about 10 days. Some of these critters are in their environment during the day and only on the birds at night, so treat at night.
Ivomec Eprinex pour on (a cattle wormer commonly sold in feed stores) is best given in a 1 ml syringe without a needle. Give 0.5 ml for large fowl and 0.25 ml for bantams. You really need two people to ensure it is applied correctly, someone to hold the chicken so you can lift the feathers on the back of the neck and be sure all the medication is dropped directly onto the chicken's skin. This will also worm them.
Ivermectin does come in injectable and paste form as well. i do not know how to dose them with these forms.
I have read opinions both ways on discarding eggs after using Ivermec. It is used on humans in Africa for river blindness.
Here is a university writeup on the subject:
http://ohioline.osu.edu/vme-fact/0018.html
I've read of a few ways to dust chickens. Put one in a pillowcase with sevin in it, leaving the head out of course, and kind of shake her around. Or make a "powder puff" out of Sevin and something like an old nylon stocking and "puff" it onto them. I've also heard of using a shaker can such as a Parmesan cheese shaker can. I've never tried any of these; I use Ivomec on the birds, and Sevin and permithrin spray in the coop. I like the permethrin because it is a spray and can get into cracks in wood, such as their roosts.