Once cured, modern paint - including model paint - is considered safe and can be disposed of in ordinary household refuse. Therefore, simply let paint dry completely and then toss it. This applies to rags, brushes, or jars. As for chemicals, I generally use them rather than throw them out. If I have something like dirty paint thinner, I just put it in a scrap can without a lid and let it evaporate. Once it's dry, it gets tossed.