Please try donating your left over paint first to local theater groups, sign painters or schools. Latex paint is not a hazardous waste which means it can be put out with your regular trash after it has been solidified. This can be accomplished by simply removing the lid and allowing it to harden, mixing it with sawdust, kitty litter or rags. Do not replace the lid so the hauler knows all liquid has been absorbed.
Save oil-based paint for a household hazardous waste collection day. If none are scheduled, you can mix it with an absorbent material like sawdust, clay-based cat litter, or rags. Small quantities can be mixed in the can. Larger quantities can be mixed in a box with a plastic bag lining. Stir with a stick until all the paint is absorbed. Mix outside away from pets and children and be sure to wear gloves. Avoid this procedure if you have chronic respiratory problems. Once the paint is completely dry, put it in your trash.