Many/most communities have a hazardous wast collection program, check with your city/town. Don't throw it in the drain, on the ground, or in the trash.

I get multiple use out of my thinner by having at least two containers, Old and New/fresh. The old is a closed container that sits for long periods of time and that lets most of the solids to precipitate out, settle to the bottom. I sometimes then transfer this clear/cleaner thinner to a new container for future use in the first wash of brushes. Eventually I will take the oldest stuff to my township's collection process. We have two or three times a year that the township provides for proper disposal of this type of fluid, and other hazardous waste.

Our county has hazardous waste drop-off stations in several locations through out the summer. That's when I dispose of mineral spirits, paint strippers, etc. For used mineral spirits, I store it in a coffee can (either metal or plastic) with the lid on till it's time for disposal. I also label the contents with permanent marker for the hazardous waste drop off center.