First, some exciting (for me) news - Alyssa has just told me that she's sold my first novel, Faithful, as part of a 2-book deal to Puffin. Needless to say, this is thrilling - the culmination of many years of effort. And hopefully only the beginning of a career that I love, love, love. You know, it's true: if you do what you love, you'll find success, because your heart will pull you through the hard times when your head yells at you to quit.
So, back to characterization. Last time I mentioned one technique - scrapbooking. There are tons of others, but one I especially like is this: envision your character's bedroom. What's on the walls? Posters of rock stars or athletes or Einstein? What does your character have on his/her bed - a spread? Stuffed animals?
And what's in your character's closet? The stuff we hide from everyone tells a lot about us: is his closet a mess or is it neat? Are there old toys in there? A hockey stick? Does she take her laundry in to be washed or does it end up in a heap? How about old papers that she doesn't want Mom to see? Or a diary?
These are the personal details that turn a character into someone recognizable. Maybe the closet door still has a height chart that your character has long outgrown. Or maybe the closet is a trunk - because you've written a novel set in 1840.
You can use the scrapbook technique on your character's room, too - create a picture with cut-outs from magazines. These physical details will start to make your character feel more real to you, and then to your reader.