Do not use glass jars for storage

[nachoman]

When my paint jars get stuck, I turn the hot water on in my faucet to a slow stream. then lay the jar on it's side in the sink with the lid to the jar directly under the stream. You want the hot water stream to hit on the lid and not on the jar (it's okay if some splashes on the jar). After a few minutes, the lid comes off rather easily with your hands, or you may need a pair of pliers to help grip the lid. If it does not break loose easily, put back under the water for a few minutes.