The best advice is to work on yourself first so that you set a shining example for your children (honesty, kindness, patience, calmness, and whatever other traits you feel are important). While you want to be on good terms with your teenager, you know that parenting isn't a popularity contest. Your teen already has friends, and although you can decide to be a friendly parent, you're still the parent. Don't let your job, shopping, friends, Internet surfing, etc., prevent you from being available when your teenager wants to talk, even though it might be inconvenient. Focus on what your teen says and don't jump in with advice. If your teenagers share information about a problem, don't yell at them. Try to stay calm and talk to them about suggestions to solve the problem. You're the adult and you've got more experience and wisdom than your teens, but you have to hear them out first.
One of your goals in raising teenagers is that they'll grow into responsible adults. Hopefully you worked toward this goal when they were children, and encouraged them to take care of their own belongings; but now that they're teenagers, you can take another step back. Allow your teen to schedule his appointments or arrange her babysitting schedule. Realize that peers are very important to teens and try to make sure that the friends with whom your teen hangs out are stable, decent human beings.
When you have something important to say, especially when it concerns your values, don't be afraid to mention it a number of times (but keep it short; no one likes a lecture). Even if you're sure that your teen is tuning out, your words are inscribed in the hard disk in his head. Finally, remember that minor issues don't have to turn into fights. You can ignore a lot of what bothers you about your teen's behavior, too.