1. know about music. for some reason (and i think they may actually be on to something), when high school students are trying to figure you (and each other) out i've often been asked "what bands do you like?" play it up...even if they've never heard of the musicians you're talking about. most teenagers are into music, so if you're into music, even if it's not their music, they have a way to identify with you.
2. a friend from church taught me once that you can start a conversation with anyone by asking where they are from, what their family is like, and what it was like to grow up there. switch up specifics and you get down to the point that people, teenagers included, love to talk about themselves. let them and they'll understand that you care and are interested in their story.
3. don't let them intimidate you. at the start of my teaching program i thought high schoolers were close to insane. they stood up in a silent classroom and started yelling, one told me he wanted to commit suicide, plenty cried, they'd put their headphones in and pretend i didn't exist...but my dad explained that when you're in high school you act tough. high school (and being a teenager) is tough, so they try to act tough to get through it. be tough but honest, be tough but straightforward, be tough but "chill" as one of my moody high schoolers said of my teaching.
4. finally, never ever forget to challenge them. there are too many people who are intimidated by them, too many people who give them the satisfaction of the shock factor, too many who do their very best to stay away from their strange culture. if you're gonna be in their lives, challenge them to be better. challenge them to look ahead. challenge them to think beyond their overwhelming emotions. challenge them to challenge the world, to never give up, and to be even greater than they were planning on.