They believe their feelings, so new and personal, are unique. They'll learn otherwise-without your help. And never imply that their feelings don't matter or will change. Because teens live in the present, it doesn't matter that they'll soon feel differently. State facts instead of opinions when you praise or criticize.
Teens are sensitive about being judged-positively as well as negatively.
Be generous with praise. Praise your child's efforts, not just accomplishments. And don't comment on the person. Set reasonable limits. Teens need them.