As much as it may seem like it, a teenager is not a wild horse that needs to be broken. He resents it when the tiniest things he does are scrutinized and acted upon. Did he walk in the door at 12:10 a.m. when his curfew was 12? Odds are that many of his buddies are still out there, so he's going to feel like a child that you got mad at him for the minor violation. Did she scratch the car? Whether she admits it or not, she already feels stupid about it. No need to ground her. Is it driving you nuts that his bed is unmade? Close the door to his room so you don't have to look at it.
Teens have a hard enough time understanding themselves and the world around them -- and if they have to watch their every step, it's going to get ugly in your house. Allow a little leeway to learn and make mistakes. It's counterintuitive, but you're more likely to get good behavior if you remove a few pages from that household rule book.