What I do is recycle it. I first put my dirty thinner in a large jar which will function much like a settling pond on a water filtration plant. After a day or two the pigments and other particulates settle out into a sediment on the bottom of the jar with the thinner above being quite clear. After this jar has collected enough clear thinner, I carefully decant the clear thinner off the top into another jar, taking care to not disturb the gunk at the bottom.
From this second jar, I now have recycled thinner that I can use use for any cleaning needs that I have. After a while the recycled thinner typically takes on a greenish hue, but this color tinge doesn't get passed onto any brushes and equipment cleaned with it, so I don't worry about it. I typically do use fresh thinner for thinning paint.
Since I use a Testor's cleaning station for cleaning my airbrush, so the only real thinner loss is whatever elevaporates or is picked up by a paper towel after cleaning. Using this system, I have been been recycling the same quart of thinner for now about six or seven years.