Many of the products in your garage are toxic and should never be released into the environment and that goes for old house paint too. I'll give you a trick, an idea and the legal way to dispose of old paint.
Let's start with the trick on how to throw paint in the trash? You and I both know that is illegal to throw paint cans in the trash. But it is ok to discard dry paint into the trash. The easiest way to dispose of paint cans is to put them inside the trash. But only if they are dry. But how do you dry a full gallon of paint? Easy, just mix the paint with dirt in the paint can or a different container. Leave the can open and place it in the sun. The dirt will absorb the paint and it will take more or less 24 hours for the mixture of paint and dirt to become a solid. Once it's dry you can place it in the trash.
Here's an idea to get rid of your old house paint. When you hire a painting company, ask the painters if they can do you a favor of taking the old paint cans away or if there is any good paint if they can use it in other future jobs. Another idea if you have good paint that you don't want you can use it. For example you can paint your basement or the backside of your fence.
But the most common way to dispose of your old house paint in the Santa Barbara area is to take it to a recycling center in town. Here is a link where you can drop of your old paint.