The best way to help your teen resist negative peer pressure is to help him/her develop a strong sense of self and the confidence that will enable him/her to feel comfortable saying "no." You can start this process by making sure you develop good communication with your teen, and show your teen that you respect his/her opinions and thoughts. Also, help your teen figure out who he/she is by making your boundaries and rules of behavior clear to him/her while asking him/her to think about how he/she feels about important issues. You can do this by asking your teen direct, yet nonconfrontational questions, such as, "How do you feel about cyber bulling?" By discussing these issues, you're not only setting up good communication with your teen, you're giving him/her the opportunity form opinions about tough issues before he/she is pressured into acting. Sometimes teens will go along with negative peer pressure simply because they don't know what they think. If you give your teen an opportunity to clarify his/her own boundaries, he/she is more likely to hold to them in the face of peer pressure. A similar tactic, which is also valuable, is to role play difficult situations with your teen. This will help him/her develop ways to say "no."
In addition to helping your teen understand your and his/her boundaries, you can also help your teen develop self-esteem. The more confidence your teen has, the easier it will be for him/her to say "no." So praise your teen when he/she has a true achievement and encourage him/her when he/she is working hard. Also give him/her plenty of opportunities to grow his/her own abilities, in either sports, hobbies, or some activity he/she enjoys. Feeling successful will help your teen feel good.
Some practical ways you can teach your teen to say "no" is by suggesting he/she either makes light of his/her "no," or suggesting a different (better) idea.