About commandment two, you need to be careful with the 'single trait' rule because you run the risk of creating a stock character. This isn't to say that stock characters are always bad - they can be useful, and the simpler your writing style is the better it is to use them since stock characters compliment easy reading. At the same time, you have to remember that people [i]are[/i] quite complicated. Our personalities shift depending on prejudices and comforts, we change with time and through experience, and the way we imagine ourselves is not always consistent with our actions. We are naturally inconsistent. I suppose you can imagine personality as an average of our behavior, or a line plotted on a graph of our actions.

What you need to do if you want to make good characters is try to be honest with yourself about what that character would do or think in any given situation. Hell, you really don't need any commandments if you can master this. Dr. Manhattan, after all, breaks the first commandment in the extreme, but the author's ability to extrapolate how his abilities would effect his behavior makes him an interesting character. 

Really, the most important thing is to never make a Mary Sue. Don't create a character who is just an avatar of whatever random shit you think of. Develop them instead, and see what you can learn about them in the process.