First: how I find books to read. Certain blogs are particularly useful in this regard: Boing Boing, Bookslut, and The Whatever* all have posts discussing the latest genre fiction, and Bookslut also provides a decent filter for lit fic. I do rely to some extent on Amazon's aggregated data, but for books that pop up in the "people who purchased X also liked Y" field, I do a little Googling to see if the gist is appealing. The SF Novelists Blog also has pointed me in some interesting directions. I've blogged all of the new books that I've read this year, so you can check out the stuff I've found here.
I've also been revisiting some of my childhood favorites lately, so I'm in a good position to make YA recommendations. To begin:
-- Obligatory Potter & Pullman picks: Both get worse as you go through, but they're mostly satisfying and of comparable quality to the stuff I used to devour. With that in mind:
-- The Pierce Duo: Meredith Ann and Tamora, that is. The former's Darkangel trilogy is SF in F clothing and keeps things in shades of gray more than one would expect of what appears to be a fairy tale with vampires. The Firebringer trilogy didn't wrap until I'd aged out of it, but the first book of it was very good. Tamora has been much more prolific than Meredith, but I can vouch for the Song of the Lioness quartet, and her later books sound similarly doughty.
-- The panoply of genre books that you must read when you are twelve or thirteen because they are enjoyable but, objectively, trashy or terrible. If you read any of this stuff as an adult it will only disappoint you. I'm talking about Lackey, McCaffrey, some Norton, Rawn, Heinlein on his off days, Asimov's fiction, and any of those interminable epic fantasy series. Get it out of your system, though; otherwise you may end up one of those people who is 25 and still thinks Terry Brooks is a good writer.
-- Good writing: There were a lot of books I read as a middle schooler for fun that I enjoyed at the time but learned to hate when reading them years later for a class (parsing every sentence for symbolism tends to suck the fun right out of a book). This is a good time for young people to read classics, genre and otherwise. It makes them feel smart for reading above their level and will assist them in later years in picking up references. Read Steinbeck, Hammett, Tolkien, early Heinlein, Orwell, Vonnegut, Bronte, mythology . . . . One book that I have always been glad I picked up as a youngster is Lamb's Tales from Shakespeare. That's more children's than YA, though. Junior high is old enough for real Shakespeare and other plays.
Bizarro note: Right now the best YA fiction recommendations come out of Jezebel's Fine Lines feature, in which they revisit favorite books of their youth. Most of the profiled books are decent, and the comment threads are stuffed with further suggestions.
* Who I feel sort of weird about linking to, since he is ticked off at me or something and refuses to approve my registration for his discussion forum. Forgive my sins, Scalzi!