Try to write about people who are from a region and culture with which you are familiar. People from your home town might sound pretty boring to you, but every town has parts of its culture that make no sense to outsiders. I live in Lubbock, Texas. Anyone from Lubbock right now would love to see a ballgame, picnic or parade rained out -- because it would mean it's raining. (It's been very dry.)
So, think about the people that you like to spend time around. What draws you to them? What are the idiosyncrasies that make them interesting?
Another important character building exercise is determining how the central conflict concerns the character and how it will bring them into conflict with other characters. My older kids have been watching the TV series Stargate SG-1 the last few weeks. The central characters of the show each bring their own viewpoint to the conflict with their enemy. "Jack" often sees a military solution to every conflict. "Sam", with two doctorates to her credit, goes for a scientific solution whenever possible. Teal'c understands their enemy best, having served them for his entire life. As an archeologist and anthropologist, Daniel often sees the perspective of the people they come into conflict with and seeks for a way to negotiate a solution. Their differing perspectives often put the team in conflict with each other, despite their friendship.
Another important character building exercise is to determine the character's internal conflict. As mentioned before, the conflict in Stargate SG-1 is externalized in many ways, but it also exists in each of its characters. Since Daniel has no military training, he often serves as the moral compass for the group. However, he is also desperate to reclaim his wife from their enemy, which causes him to often want to set his ethics aside.
When you are coming up with characters try to think about their "Arc". In other words, how will the character change over the course of the book? It doesn't necessarily always have to be improvement on the part of the character. Consider the characters in the Lord of the Rings trilogy. (Some spoilers here...) Aragorn grows from a "Ranger" to a King. Legolas and Gimli grow from enemies, to competitors, to friends. Sam grows from a timid eavesdropper to a brave warrior. Gollum is ultimately consumed by his obsession. Frodo succeeds in his quest, but is left so scarred that the Shire becomes a foreign place to him.
Your characters need to have faults in order to come off like real people. Be honest, there really isn't anyone that you know well that you can't find some fault with. In Broken Toys: China's Song, the toys are just what the title implies -- broken. Some of their scars can be seen from the outside, others they keep hidden deep inside. This is especially important in Christian literature. Psychologists like to talk about cognitive dissonance -- the idea that we aren't always the kind of people that we say we want to be. Characters who are torn between warring aspects of their own personality behave unpredictably, which makes them interesting to read about.
Be warned, that your characters may take over the book from you.
In Broken Toys: China's Song, I originally envisioned the china doll as a minor character--not much more than a symbol for how much stock we put in the way we look and how vain that is when our lives are so very fragile. But I had one problem: why would anyone made of porcelain ever leave the attic? If China was afraid of being broken, wouldn't she stay forever on her doll stand? Surely, someone who was this desperate to get her song back was made of some pretty strong stuff. As I worked through the first draft, I found China's character growing stronger in each scene. She, in many ways, becomes the conscience for the group of toys. In the final edits, she pushed her way past the leading man into the title of the book.
One final note, in the interest of full disclosure: Stargate SG-1's pilot Children of the Gods contains nudity. How much nudity depends on which version you get. Please preview your copy before showing it to anyone else. We have not shown the unedited pilot to our children. Parents may also be concerned about the language and the violent content of the series in general as well. Our older children are teenagers and we also have a "TV Guardian" which blocks some of the profanity.