It seems that the biggest responsibility parents and adults have with adolescents is to make sure they know when they have made a mistake and what to do about it. Adolescence is also a time of developing compassion and it is just as important to reward the good deeds they do, large and small, as it is to monitor mistakes. It is easier to understand that adolescents are trying limits when they go too far than it is when they are doing what we want. But make no mistake, adolescents are trying out behaviors in lots of ways to see what works for them. Letting them know when they are doing right is as important as letting them know when they are doing wrong.
As adults, we may have to change our habits to make sure we catch these good deeds. Often, our sensors are set on mistakes and we let the good things slide by unnoticed. The best way to change this is to keep track. Set a goal for yourself to catch them doing something right once or twice a day and tell them what they did right and why you think so as soon as you can. Jot these into a Good Deed diary and then challenge yourself to match them! Adolescents learn by example, so let them see you doing good as well. I will bet your goal increases rapidly and you can feel good about yourself, too, because we usually don't track ourselves either.
One way you can use the information you are recording is on special occasions. For birthdays, holidays, or other special celebrations, put this information on cards or letters to your adolescent, telling them how much you appreciate them and what they do, how proud you are of them, and how much they have inspired you. Be sure and give specific examples so they will know that the incident wasn't just complimented and forgotten in the moment, but that it had a lasting impression. This reinforces to them the importance of the behavior and will encourage them to repeat it. If possible, nominate them for a prize so that their family and community can help reinforce the good behavior. But I will bet that the most important thing for the adolescent is that you took time to notice and comment on what they did.
Keeping this perspective will also help you to not over-react when mistakes are made and will help you keep a balanced picture of the adolescent in your mind.