1. The BIGGEST thing I think that helps is to have a stable home environment. A stable marriage and a stable parent/child relationship is key, as kids tend to act out their insecurities. Read more articles on this blog on how to achieve these things.
2. Family Traditions and Regular Family nights/outings- Your child needs to feel like they belong to a group. If your family is not actively doing the above things, they will seek it in their peers. (Some time with friends is good, but you want them to be more bonded to the family than to their friends).
3. Listen before judging. Kids need to feel accepted, respected, and loved, if they don't, they will also look for acceptance in their peers.
4. Teach them your values, religious or otherwise. Children tend to flounder unless they have a system of values they've been taught. If you are religious, teach them to have a personal relationship with God. This will give them strength and help them to not feel alone.
5. The Touch Love language- Make sure you are hugging your kids (girls and boys) and giving them pats on the back or wrestling if that is what they like. Even if your child is a teen, they need touch. If they don't get it from you, they will seek it elsewhere.
6. Develop their talents- This will help self-esteem, a necessary component in fighting peer pressure.
7. Keep them busy- As they get to be teens, they will need to fill their time with good things, so boredom doesn't drive them to bad things. I've found that a part-time job when they turn sixteen really helps them to stay focused on good things.
8. Show and tell them you love them always.