At some point, I started taking personal character traits, exploiting them until they no longer represented (only resembled) myself, and then formed these traits into various characters. Call me schizophrenic, but it seems to be an approach that works.
If you having trouble with characterization, try complicating that particular character. Think of a trait you can identify with, perhaps even a trait about yourself, and imagine how giving the character that trait would make him or her more three-dimensional and believable.
Another perk to using this method of characterization is that you are intimately connected with your characters without even trying to be. One of my main characters is a bit germaphobic, which isn't difficult for me to describe because I myself cringe before touching a public door handle.
I'm not suggesting you make your characters autobiographical, because they'd all end up the same. What I am suggesting is that you use traits unique to your own identity to enhance your characters.
What are your thoughts about this method of characterization? Have you tried it before? What are its strengths and weaknesses?
Do you have any other suggestions for strong characterization?