Firstly, think like a teenager (you were one once so not that difficult)
Avoid confrontation. Don't lay down the law, be a bit more subltle by making suggestions that will help you and the teenager. Don't ecpect instant results. Their first reaction will be to do the exact opposite of what you say so let your 'sensible' suggestions have time to be absorbed and accepted.
Never use an ultimatum, you will lose!
Never cut communication lines. If you are not talking you will never have any influence.
Teenagers are usually untidy, messy, rude, uninterested, lazy unless they are getting there own way. Accept this, they don't stay teenagers for long.
Flouting curfews. Back-talk. Using their knowledge of technology against us. Problems adhering to chores/rota. Comparisons with friend's parents. Could go on forever here...
@ Julie - might I suggest a possible solution as originally adopted by my Bro-in-Law. He removed the door to my niece's room. That way every guest using the loo saw the state of her room. Sometimes embarrassment (some might call it passive-aggressive) works wonders on teens.
Rolling the eyes, in complete disgust or dismay.
This also turns out to be one of the biggest killers in any relationship, especially a marriage.
It says, "I give up on you," or "I'm gone, outta here." See ya'. It tells the other person they are not significant, or worse, "no longer" significant.
Texting at the dinner table.
Might seem off the wall but I'm curious. I study relationship dynamics and I was wondering if you could tell me. What's one thing that "gets to you", like a frustration, with your teenager or raising your teenager?