Teens and twenty-somethings are uniquely sensitive to desperation in general, and especially to the kind inherent in 'hard sell,' traditional advertising. One way to get past their radar and win that spark of amusement, that crucial momentary connection, is to allow a sense of irony into the core of the campaign: to make ads that mock ads. This can dismantle their radar for phoniness by being preposterously and intentionally phony.
A piece of cult humor as yet untapped by advertising creatives: ENGRISH. Yes, that lovable, often hilarious Japanese ad and packaging copy, written in English but that goes all the way to production without being edited by an English speaker. http://www.engrish.com/
Happy smile, Lucky smile, Joy smile! (your teen brand here)