How do you get your children to follow the rules?

How do you get your children to follow the rules? I have an 8 and 4 year old and they are good kids really, but they just push so many buttons. They like to waste things....I put a rule in place that wasting is over but they still do it! They waste food, shampoo, toilet paper, toys (tearing heads off barbies)....I don't let them get more food till the first plate is finished, I ground them for wasting things in the bathroom (even went so far as breaking their favorite toy in front of them for it!), and stopped buying them new toys.... I don't know what else to do! HELP!