D.D.
1) If you combine all of your old paints (like kinds latex with latex ect)you will get a great primer.
3) Paint the left over paint onto a piece of cardboard and let it dry. Then leave the empty can to dry over night. These can both then be disposed of with your local trash hauler.
4)Join a local Freecycle(tm) board and offer your left over paint to someone else to ues. You can find a group for your area at http://www.freecycle.org
5) See if your city has a hazardous chemical drop-off. (I listed this last because it should be the absolute last resort).
D.
SAHM mom of two (18 and 4). HUGE advocate for using our resources wisely and portecting our environment.