It's easiest to get your teen to talk to you when you make yourself someone they want to talk to. The way you open conversations with your teen and respond to conversations he/she starts with you has a lot to do with how deeply and often your teen will share with you.
When you want to talk to your teen, you're more likely to get a detailed answer when you ask open-ended questions, rather than yes/no or one-word answer questions. So instead of asking "How was school today," you can ask, "What was the best part of your day today?" Also, make sure your questions are specific. General questions lead to general (uninformative) answers. You should pay close attention to information your teen provides without communicating -- what posters are on his/her wall, what songs are the iPod, etc. Using this information, you aren't relegated to asking, "What movies do you like?" Rather, you can ask, "What do you like about those vampire movies?" Even if it's a topic that doesn't interest you, if it interests your teen -- ask him/her about it. Lastly, when your teen answers -- listen! Once you start asking follow-on questions that touch directly on your teen's previous answer, he/she will see that you really listen to what he/she says. Nothing will encourage your teen to open up more than knowing you're really hearing what he/she says.
Once your teen does start to initiate conversations with you, make sure you're not reacting in a way that discourages him/her from continuing. For example, it's common for teens to worry that parents will overreact when they share information. If the most interesting event at school that day was that two other students got into a fight, your teen might not want to tell you if he/she fears you'll take that opportunity to lecture him/her about not fighting. Not every story or moment has to morph into a "serious talk." Sometimes, just listen. Don't teach. Don't fix. Just listen.