The teen years are tough, just ask any grown up. You'll hear all sorts of stories about the many horrors they've endured and how their teen self esteem was easily crushed, only to rise quickly like a sphinx from the ashes. This is the typical when it comes to self esteem and teenagers.
One day they're feeling like the scum of the earth and the next day they're going to be the next president of the United States. These self esteem ups and downs are normal, as teens react dramatically to most everything.
Sometimes, though, a teen may suffer from low self esteem for a long period of time. In many cases this is due to things like acne, break ups with significant others, and even unintentional comments made by adults whom they look up to. While a dip is nothing to worry about when it comes to a teenager's self esteem, a low dip in their self esteem that doesn't bounce back up within a reasonable period of time is cause for concern.
One thing you can do if you think your teen is suffering from low self esteem is to give them as much positive feedback as possible--without gushing. Teens are highly suspicious of most any adult and will disregard anything an adult says that seems "over the top" to them. In other words, they know when they're being patronized and don't like it. So make sure your positive comments are realistic. And don't expect them to acknowledge your comments or that they're having an affect. Just continue to offer positive comments on their behavior whenever possible. Teens will listen, they just won't let you know they're listening.
Some other things are to give them many chances to have new experiences and to encourage them to take part in activities they've shown an interest in or have a track record of success in. Keeping teens busy in activities that give them a lot of opportunities for positive reinforcement will get their self esteem up and keep it up.
However, if you think your teen is suffering from seriously low self esteem, don't hesitate, but get them professional help as quickly as possible. The teen years are ones that will affect a person's self esteem and self confidence for years to come, so whatever you do, get your teen professional counseling during what can often be turbulent times that can damage self esteem for decades to come.