I've read many books on writing, and they often suggest writing out your characters' attributes, either as a list or a few paragraphs. I do think this is a great idea to keep things straight--characteristics like age (a specific birth date even), physical description, brief bio, dominant personality traits, etc. I have these things scribbled throughout my notes on Finding Angel, and it's one of my goals to compile everything into a file this summer so I can reference things more quickly.
Recently, however, I have discovered a fun and effective way to get to know my characters and really cement these things in my head. I'm writing short stories based on each of my characters from the first person point of view. Most of them will serve double-duty, as the main character of each story is interacting with other characters from the books.
The coolest part of this is that I'm discovering things about my characters I never knew before. If you're not a writer, you may not understand that statement. But, my characters have become real people in my head. And as such, they are complex, and even I can't know every detail of their lives from birth to death. I know some specifics, but many times it's more of a general idea. For example, one character has a rebellious streak because he has an overbearing and controlling father. Well, I'm working on a short story right now that describes one of his rebellious acts. It's given me some insight into why he behaves certain ways, and is helping me keep him consistent in my second book, Seeking Unseen.
The first short story I wrote like this actually gave me an idea for an element to work into Seeking Unseen, that will have a major effect on one of the characters. And, I must say, I think it's a pretty cool kind of magic :). Now, I've got this story submitted to an online magazine, and if it gets accepted and you get to read it, the element I came up with is not actually found in the short. It's a side-shoot idea that will only be in the book. But don't let that stop you from reading the story! I will, of course, post if it gets accepted anywhere :).