One way to add to the depth of a character is to add back story. But, when and how you do this is a tricky business. You don't want to overload your readers with a sense of too much-information-at-once. You want to add it when it is right and only then. Sprinkle a little throughout the story.
That being said while reviewing my manuscript I've realized that I really don't have that much back story included. I really need to add more, in a sense adding more depth to my main character. So, as I've been going through reading my manuscript I ask myself - Is this the right place to have a memory float in? If so, I think about what feels right at that moment and write in that memory. Since my manuscript is in the first person it is best to have my main character remember past events. Hopefully, this will add more depth to her and add a sense of understanding for my readers.
Back story doesn't have to be a huge memory several paragraphs long. It can be short and sweet, a sentence or two. Anything that adds to the characters past and helps the reader build both a relationship with the character and an understanding of why your character acts the way he/she does. And, that's what I'm going with. Short and sweet. This is especially good since I'm already over the 80,000 word mark on my manuscript and I'm also working on adding more depth to other characters at the moment as well. I don't want to end up with a 150,000 word young adult novel. The chances of selling that to an agent are slim.
Your Turn -> How do you handle back story?