I asked the same question of a work colleague whose dch are in their late teens. She said 'Be prepared to listen to them, rather than just lecturing them all the time.
My dch are nearly 15. I've found it hard to be laid back about dch suddenly being invited to tea or to Sunday lunch, or 'X wants to stay tonight, OK?' but I've forced myself to say 'Yes' more than saying 'No' and mostly it's fine.
My DS is a bit silly still, but DD is more grown up, and some of their friends are quite mature & I treat them all like adults & we have a laugh. The advice about being unshockable is good - if they feel they can say anything in front of you, they're less likely to hide things. They tell me things about their friends (eg X has tried drugs)& we discuss & I can give a bit of advice, but I do try to listen & not be seen to judge.
Last week DD asked if I minded her getting her belly-button pierced. My instinct was to shout 'Yes, I bloody well do. There is NO WAY...' etc. But I didn't. I pretended to think about it, and eventually said that I didn't mind, but could she wait until she's 15? She said OK. Then I said that when I'd looked into it for myself they'd said it would hurt like mad for about 20 seconds, OK? After a pause she said that actually, she thought it was a bit tacky & wouldn't do it after all. Job done! (I've no doubt that if I'd said 'No way' she'd still be banging on about it.)
Don't worry about the teenage years - if you do a good job now, teaching right from wrong, teaching them to think for themselves, making them feel secure etc, you'll be fine. I find having two other 'adults' to share life with wonderful.