Research your book's demographic. That means you should be diving into social networking sites, according to Writer's Digest's tips on boosting your teen novel's appeal. Listen to what teens have to say, note what makes them tick, what they are interested in and what is popular in their lives. Pick up on the way they speak to one another and note any trends in their discussions.
Look at the current trends in the world---economic problems, teen deaths as a result of drunk driving and other trends may provide a scenario to develop your drama around. According to Writers Digest, being relevant can help a YA novel. The global perspective within your book may pull reader's in, since people will be able to connect with the characters who are going through the same things they might be going through. Even if your book is based in fantasy, many of the same real-life problems can arise.
Develop characters with which teens can connect. According to the website Women on Writing, the author who writes teen books must know his character inside and out. The teen character should reflect actual teenagers---she should have believable characteristics that are engaging and empathetic, even if they aren't necessarily good. For example, Stephanie Meyer's best-selling book "Twilight" revolves around a vampire. Though vampires aren't typically considered "good guys," teens connected with the vampire's background and belief system because Meyer illustrated him in such a way that readers could connect.
Write in a voice that teenagers can connect with---use colloquial, believable language. Have your characters speak in a voice that reflects the teen population at large. Long, fancy words probably won't be a big hit with teenagers. Study how people speak on social networking sites and keep this in mind when writing.