Make sure that you follow the directions on the can exactly. PAM is nasty stuff that not only can kill mites but reptiles and even humans if the directions aren't followed.
You should put your snakes on paper towels instead of what ever substrate you use... this does 2 things.
1. It allows you to see mites better
2. prevents mites from hiding in the substrate
You should remove as much from the cages as possible the less that is in there the less there is for mites to hide on/in.
Keep the cage hospital clean, and try to keep cross contamination to a minimum.
You should soak your snakes twice a day in warm water (water temperature in the mid 80's) with a drop of dawn dish soap.
If you use half log hides or anything that is wood they will need to be baked in the oven or simply thrown away. (oven temperature 350 for 20-30 minutes This is far below the flash point of wood so it is safe)