First of all, read the books your teenagers read. Talk to them about the books, omitting any comments that contain the words, "duh", "lame", or "obviously written for young adults". When they put on some music in the car, bob your head a little to the beat and pretend you know the words, or at least that you totally get the message of the song. Understand that your teenagers do not think that stopping at DunkinDonuts is a waste of money. Frappes and mochas and all of those other drinks that weren't invented yet back when I grew up while dinosaurs roamed the earth are totally worth four or five bucks each. If you want your teenagers to have fun with you, then go ahead, stop at McDonalds! You don't have to actually eat the food, but a quick way to their hearts is to let them order a thing or two from the dollar menu!
And, most important, after reading the books they read, when those books are made into movies, take them on opening night! Shriek with delight and anticipation, right along with them, and when that movie is over, clap your heart out. It doesn't matter if you loved it or not, you had fun with your kids, and they think you are totally cool!
Now that I have written this, let me say this: thankfully, I don't have to pretend to enjoy any of it, because I like books, I like movies based on books, and I love hanging out with my teenagers.