Retreat the tubs/rack/enclosure with PAM every two weeks.
Keep a stash of treated and dried paper you use for substrate handy so that when your snake makes a mess you just replace the soiled paper with clean treated paper.
Reptile Relief can be re-applied every three days. I don't spray the snakes, I spray a white paper towel and wipe the snakes with it. That way you can get into the loose skin under the snake's chin where mites love to hide, and the dying mites will show up on the paper towel.
Continue to treat for mites for 30 days after you find the last one, as an egg can take a month to hatch.