1. Start with a character outline. Character outlines ask some awesome questions that help you guide your thinking toward who the character is, what they stand for, what their strengths are, and what their quirks and weaknesses are. I will be posting some links to some good ones in the Editor's Picks section.

2. Write some short stories about your characters. No, not just your main, ALL of your characters. Or, at least, the ones who will play an important role. Put them in some interesting situations and see how they will react. Develop their strengths and weaknesses this way. The situations don't even necessarily have to match the main story they are involved in.

I once read a book (for the life of me, I can't remember the name) where a woman was plucked from the modern world and placed in a Jane Austen novel setting as she fell asleep reading a Jane Austen book. I loved it! It was so much fun to see how someone from the modern world would see that sort of time frame and setting. I think it is also an amazing way to develop a character.

One warning that goes with this, however. Make sure that you don't have the character reminisce about these side stories in the main story, unless there is some way to get the material in there for the reader to understand.

3. Be sure your character can function independently. Have you ever seen those people who seem like they are incomplete unless they have a significant other? Don't let your main character be that way. Your main character needs to be able to stand alone, even if the thought of doing so terrifies them - they need to be able to do it.