Be consistant. I know that sounds easier than it is but it does work. Unfortunately, kids this age think no is funny. My boys did the same thing. They have to know you are serious. Don't laugh. They'll think it's a game, then. If something is wrong one day, it has to be wrong the next, too. I don't know what else to say, eccept to warn you, if you laugh now when you are trying to dicipline, they won't take you serious and if you think it's bad now, wait until they're ten. It only gets worse. My youngest thinks it's really funny to talk back and he tries to push the limit. I've had to get really tough. It's better with my oldest. He doesn't fight it when I say no to him, So far. lol. Good luck!!!!