Acetone (aka nail polish remover) takes out the resin like no other. Fill it, shake it around a bit, and pour it out. Repeat a few times until most of the resin's loose. If it isn't completely clean yet, soak the piece in HOT water, then repeat the acetone wash a few more times.

Remember to wash it thoroughly after the acetone with warm water, or else you'll get some acetone vapors when you go to hit it.