Yes, we all love a book with a good, strong voice. But...what does that mean, exactly? Let's say you are writing in 1st person POV. That's certainly the easiest way to create voice (though not to say it is easy). How do you create this elusive voice? My first answer would be to really make sure you know your character. Is s/he strong willed? Funny? Sarcastic? Sweet?
Spicing up your narrative with the occasional dose of humor (if appropriate to the story) may be a good way to create voice. Funny is hard, though. If you are writing YA, don't just read everything that's out there in the genre to determine what is funny to teens today - though that's a great way to start. Watch the successful movies and TV shows that have humor that works for that age group too.
Sarcasm. A very common way to provide voice to a teen protagonist. Of course, right? All teens respond to sarcasm. IMO, there is witty sarcasm and then there is just plain mean, non-witty sarcasm. If you're doing that second one chances are your main character is going to be really unlikeable. Ask yourself, if I were to have a conversation with my protagonist, would I want to listen to them go on and on for another 200+ pages? If the answer is no, you need a new voice. Also, keep in mind that a lot of other people are trying to write similar, angst-ridden hormone-driven characters. Maybe your protagonist is special. How?
How else can you create voice? There are more subtle ways than those two. Maybe your character is just sugar and spice and everything nice. A really sweet girl, who never thinks a bad thing about anyone. So much opportunity for her to make excuses for and misread the people around her! That's her voice! TaDa.
As I said, know your character. Let him/her speak to you, and know the lens through which they see the world. How will they react? Let those things be the clues you need when creating voice. -AH