So, here goes:
1) News stories. At Alloy, they had to take clippings of papers whenver something interesting would happen, and then they'd all come in with these stories and talk about how they could become a fictional story. BREATHE MY NAME, by RA Nelson, (not an ALLOY book) was born this way. Nelson saw a story about a woman who drown all her kids. So he thought....what if you were a survivor of that? But that's not the story. That's the backstory. To get the hook and the story, he added to that-- in BREATHE MY NAME, the survivor-girl receives a note from her mother that just says, "We're not finished yet". That's it. So she launches a cross-country road trip to find her mom and find out what the note means.
So, start watching for interesting news stories, and then think outside the box about how that could become your backstory for something really amazing.
2) Something from your childhood. The editor who came up with SISTERHOOD OF THE TRAVELING PANTS, once had a pair of jeans that fit her and her two freinds, and they shared it. They weren't magical or anything, but it was a tiny grain of an idea that could be expanded upon. The pants became "magical" in that they fit four VERY different girls. You'll still need the story line and the adventure, but you have the hook-- and that's where it can become high-concept.
So brainstorm what seems like random things, and develop it into something bigger than life.
3) TITLES. Liesa mentioned that one day, very tired and out of it, she was staring at her hands and thinking about fingerprints and how unusual they were. And then she thought....FINGERPRINTS. That would be a cool title. But what kind of book would it be? They ended up developing it into a story in which a girl could touch fingerprints left behind and use that for pyschic powers about the person who left it. I've done something similar-- come up with a title first. GETTING CAUGHT came to me while listening to a Sara Mclaughlin song, FALLEN, and a line said, "I got caught up in all there was to offer...." And I thought, wow, what YA book ISN"T about getting caught up in something?
And then I thoguht...wow, Getting Caught, that would be a great title. But Getting Caught isn't really about getting caught up in something, it sounds more like, red-handed, getting caught. like Pranks. And then GETTING CAUGHT was born, about two high-schoolers in a prank war that won't end until one of them gets caught.
While writing this, I was also thinking of song titles and lines, how they can become story ideas. For some reason Kelly Clarkson's song MISS INDEPENDANT came to mind, and I thought, how would that become a story? What if the girl is independant because her mom ran away or something and left her to fend for herself? What if she's secretly paying the rent with her part-time job, and no one knows she's living all alone in some ramshackle place, hoping she turns 18 before anyone catches on? And what if someone DOES catch on?
[ETA] I forgot two more tips!!:
4) Taking anything successsful in the adult world, whether it be a movie, a television show, etc, and turning it into a YA fic. She gave the example of Sweet Valley High, which was developed based on the TV series DALLAS. I've never seen the show so I have no idea how that relates to the books, but there you have it. Also, I'd TELL YOU I LOVE YOU BUT THEN I HAVE TO KILL YOU, I know, was developed after Ally Carter was watching ALIAS and was thinking to herself, "where did this girl learn to kick butt? Did she go to spy school? And if she did go to spy school, what was that like?" and so her book, Love you Kill you, (on the NYT bestseller list!!) was born.
5) Make someone's dream come true. Think of your audience and the type of thing they would fantasize about. Harry Potter who felt lonely and unworthy, wanted to be special. So JK Rowling made his biggest dreams come true: not only did he have a great adventure, he found out he was special. THE BEST. in PRINCESS DIARIES, Meg Cabot takes one of the biggest dreams of YA's (being a princess) and makes it happen. Take someone's wish and make it your hook!
So, Do you see how a simple phrase or title or idea can quickly become a high-concept idea? They don't write themselves, and they don't come to you fully developed as an idea or hook.... but if you're open to them, they're EVERYWHERE. You jsut have to find the one you want to write.