Picture the character five or ten years ago, and five or ten years in the future. After you've created your character, it's important to remember that they need to be treated as individuals with their own needs and agendas. Don't make a character violate their basic nature just to suit the plot. If this character's actions would change your plot too much, you need to either re-examine the plot, or create a more suitable character. Allow your characters to have quirks and contradictions though.
Most people have a thousand little contradictions.
The most important advice I can give is to respect your characters. Treat them as important people, whether you like the individual character's personality or not. Remember that their feelings matter to them. What happens to them is important to them. John Hewitt is a freelance writer and an Internet strategy consultant with the OSI Group.