There is a thread on duct tape that might provide a few suggestions.
Okay, I am kidding about that.
My suggestion is, lay down the law and be ready to back it up. Explain the rules clearly, and make sure they understand the rules you set. Then they meet the standards you set or they receive a punishment. Make sure it is a punishment that you can enforce. Stick to it. It might be hard at first, but once they see you mean it, they will stay within the guidelines you set.
Any punishment should be done right then and now, not later. Don't threaten no desert at breakfast time. Don't give chances (this sounds harsh, but it cuts down on "testing").
And if you find something that works all the time, write it down, copyright it, and sell it. You will become rich. In other words, your kids might not be little angels, but niether is anyone else's children little angels all the time either. Childhood is a time for learning and making mistakes.