In addition to what tomjedrz says, it is critical that you listen, listen, listen. They will be more receptive to your advice and guidance if they feel heard and respected. Try to treat your teen as though a young adult you are mentoring at work or something along those lines instead of treating them like a child is often treated by a parent. Of COURSE you have final say and of COURSE you are NOT a friend. However, a mentor is still in a position of authority, but people being mentored are usually treated in a way that is much closer to what it is your teen wants from you in terms of treatment.
No matter how much they protest, they DO still want your guidance and they know they still NEED it, they just also want to feel as though they are being treated like the adults they ALMOST are.
Before telling teens what you think about a decision they are making, it is important to ask them what they think and listen as non-judgementaly as possible (at first). Ask clarifying questions, paraphrase (i.e. "let me make sure I"m understanding you correctly, You think. . . ") and make sure they know you heard, understand and respect their thoughts - even if you disagree.
Then, depending on how the teen's answers and the topic at hand are playing out, you might ask questions like, and what are the benefits of _? What are the drawbacks? OR you can state your opinion about their conclusions (respectfully, but succinctly). If they don't want to listen to you, you can point out that a. you are the parent still and b. you listened to them, now it is your turn to talk.
While the relationship required for this to work takes time to build, it is worth the effort and will not only make the both of you closer, but help your teen learn life lessons about critical thinking and decision making he/she can't learn while you enforce rules and dictate how they should make many of their decisions. As painful as it may be, let your kids make small mistakes now and help them avoid the big mistakes by choosing your battles to a much greater extent than you are naturally inclined to.