I say "that kind of response is disrespectful and I expect you to apologize and speak to me nicely instead." Then, after the inevitable second bout of back talk I give consequences if the behavior is not done.

As for the teeth brushing, etc. I say "you know, the most unpopular girl in my class was the one who had skuzzy teeth and smelled funny. I hope the kids in your school aren't mean to you like that."

Use peer pressure to your advantage.

Also, I use either/or..." would you like to brush your teeth before your shower or afterwards?" Give them some sense of control, and in a subtle way your expectations are slipped in there.

I have a 17 yr old, and the only rule I can get him to follow is a curfew. Good luck there...no advice to give ya!