1. Main Character Must be a Young Adult.
I say young adult, because I think you can have quite young protagonists, younger than teenagers, and a bit older, into their early twenties. This is a magical age in anyone's life. It is also a bit of a trying time for many. Socially, it is difficult for almost everyone. There is a lot of self doubt and fear. Strong emotions hit hard at this age and must be dealt with for the first time. Falling in love really hard with someone for the first time usually also happens at this point of life and can be rather mystical, magical, amazing and horrible all at once. Young adults sort of start narrating their own lives at this point. They see their lives as an adventure book where wonderful and awful things happen. And, for most of them, they hold out hope that things will get better if they hang in there.
2. Invent Some Screwball/Original Characters.
No need to go through the lengthy list of original, interesting and bizarre characters in the Harry Potter series, other than to point out how successful those books were in great part due to those extremely identifiable characters. The thing that really makes a book interesting isn't magic, werewolves or love between a human and a vampire; it's the characters. Funny, silly, scary, weird, fanatical, characters. Once you have some of those your story will take off without you if you're not careful. Young adults love to read about characters they believe are more off-kilter than themselves. Quite often they feel like the weirdo; give them someone else to identify with and laugh at.
3. Put Your Characters in Danger.
Danger Will Robinson! If your characters aren't in trouble for their lives, find a way to put them in some. I know there are some really great YA books out there where their lives aren't in danger, but perhaps their sanity, or reputations are. Something must be at risk. But putting their actual lives in danger, if you do it believably within the confines of the universe you've created, it makes the book much more exciting. When kids are young, they loved to be chased. Running away from danger is exciting. Turning and facing it is even more exciting. I'm just saying, real danger creates the adventure both kids and young adults crave - (especially if they can safely read about it within the confines of written book).
4. Create a Sympathetic Hook.
If the reader doesn't identify with your character(s) for one reason or another they won't really care about them. You can put them in danger all day long, but it won't matter if the reader doesn't like them. One great way to do this is to hurt them somehow. I know, it doesn't sound nice, but if your protagonist isn't struggling in some way (their parents are dead; their only friend is moving away; they don't know where their next meal is coming from; they're suffering from severe acne, you know, the kind that scars) it's hard for teenagers, who already don't feel like they fit in, to relate.
5. Add a Fantastical Element.
This isn't always necessary, but it helps. Almost every adolescent in the world feels powerless.
They are forced to go to school by parents who don't understand them; they have to relate to heartless, cruel peers who are out to ruin their lives daily; they have piles of homework to complete each day from 38 different teachers. It's nice for them to fantasize about having power. It could be invisibility, it could be magic, it could be sucking blood from people and staying out of the sun, but something that gives them a sense of power. It's liberating.