I keep a can (or two) around all the time and I use it on any new enclosure I put together as a preventative measure. In other words, I treat each new cage with a dose of PAM (Provent-A-Mite) to prevent/kill any mites from the start.

When using it, though, you have to follow the directions very closely. It's quite toxic if used wrong.
And yes, most mite sprays in pet stores aren't worth the plastic bottle they come in. I only have a bottle of Jurrasi-Mite on hand for use on the snakes themselves as a temp relief, since you CANNOT use PAM on any part of the animal.