I’ll get straight to the point, you can prolong the shelf life of most types of paint by using Duct (or Duck) Tape.
I’ll start with household paints (the big stuff) and talk about craft paints later.
Some people turn the tin upside down, so that the paint forms a seal around the lid, but that dried paint falls into the tin when you open it.
This prolongs the life of pretty much anything in this sort of pot, that “air dries”. I use it to seal the lid of my contact adhesive, too.
If you work on small craft projects, and you mix your own, small amounts of paint, duct tape is a great way to keep it useable.
I cover making your own paints in other tutorials, here are a couple
Any small container is OK to use to mix up your paint, as long as it has a smooth top edge. When you want to store it, make sure that the edge is clean and seal it with tape.