Lift the bag, then let it drop a short distance and catch it briskly. Imagine you're lifting up a kid, and you want to make him think you're going to drop him. It causes the particulate that's settled to the bottom and is clogging your bag's pores to raise up rapidly, while the water rushes down and out. If done right, you can drain a 20+ gallon set in under 30 seconds. It's all about catching it in a quick, firm snap. This works for all sizes, but it's hard to make a short enough drop not to splash out of a one gallon set. You can also paddle the bottom of the bag with a gentle but rapid spanking motion. Use a tool or you'll freeze your hand off, but the drop method is undeniably the best and fastest.