I was reading some posts about how people get rid of paint and thought I would share my technique. I call it an evaporation pond. It is the same thing I use for grey water at Burning Man and it works really well. Simply get some black plastic (the larger the better) and a couple of boards together and you are ready. Spread the plastic out in a sunny location and place the boards under the plastic around the edges, (stapling is recommended.) You have now created, not only a barrier from the earth, but a place to dump your wet paint. You can also clean your brushes here instead of into the sewer or septic where it will remain toxic and hard to break down. When all the paint is dry you can roll up the plastic and save it for another time.
Remember: paint is toxic as a liquid and garbage dry.
If it is rainy or not hot enough outside you can hold on to your paint untill it is. I did that last summer and when I got a chance to make an evaporation pond and dump out all my old paint I felt like I was making a Jackson Pollock painting. It was neat looking and fun, even inspirational!
Just simply opening a paint can and leaving it somewhere out of the rain for a few months works too, but you may want to add something like wood chips or straw to speed up the process.