Mites can be brought in on the animal itself, any cage decorations that you put in the cage, the substrate you buy, or even on your body. Those little boogers can be a task to get rid of too!!!
First thing to do is go to the pet shop and get a can of Provent-a-mite. To save some money you can go to Walmart in the camping section and get Repel Permanone or Sawyers Tick Spray. (Active ingredient: .50% permetherin).
Next take the snake and the water bowl out of the cage, leave everything else in the cage. Set the snake up for a nice soak. Put approx 1/2-3/4" of water in a plastic container with a rock or towel for him to rest on so he doesnt get too tired. The water should feel slightly warm to you, not hot.
While the snake is soaking spray the cage down with which ever spray you purchased. Stir up the substrate, then spray again. After spraying cover the top if it is a screen top and leave it sealed up for about a half hour. When the half hour has passed give the cage another couple of blasts of spray and let it sit for another 1/2 hour. Finally when that time has passed, open the cage up, let it air our for an hour or more. Once the cage has completely aired out and you dont see a fog in the cage, then you can put the snake and the sterlized water bowl back in the cage.
Keep a close eye on the cage and snake for the next two weeks to see if any live mites show up again, but they probably will not. Dont be surprised to see some dead mites in the water bowl and on the snake as the spray and residue from the spray will continue to kill mites for at least 2 to 4 weeks. If after two weeks you are still seeing live mites retreat the cage.

To prevent mites in the future pretreat or clean everything that comes in contact with your snake before it even gets close. I use similar spray to pre-treat my cypress mulch substrate before it even goes inside my house. Any cage decorations, bowls, or hides are either washed in the dish washer or sprayed prior to taking them in to the reptile room. With a little precaution on your part you will never have to deal with mites again!