Spider Mites seem to multiply most when the plant they are on is in a fairly arid, dry, area so spraying often with water, to increase the humidity around the plant can reduce the population. Looking on the leaves for egg masses, and squishing them or otherwise killing them will also reduce populations. An application of horticulural oil, on a houseplant I would use a Superior Oil, carefully, can also help in control.

Here is a link that might be useful: Spider Mites

thanks, i think im just going to clean out my greenhouse anyways so if i put them in my compost bin with all of the still green plants, do you think it will heat up enough to kill them? I hope so because thats what i did!

I always used to put my house plants in the tub and spray them thoroughly with COLD water about once a week for several weeks. Worked great every winter. They don't like wet or cold and the harsh spray washes them off also.