Grandpa always said - "If you are too lazy to follow through on what you say then don't say it."
When you tell your child to do something (or not to do something) make sure you stick to what you said. If you don't then your child will learn that what you say is negotiable.
If you say "No, you can't have any candy." and your child cries or throws a fit do not give in and say, "okay, just one." You are teaching your child that things are negotiable if they throw a tantrum or keep bugging you.
If you tell them not to touch something and they do it anyways, don't ignore it. You need to get up (hence grandpa calling it lazy if you don't), walk over and repeat that the child cannot touch something.
Stick to your guns or your child will constantly test you looking to negotiate.