Tim Saunders gave some excellent advice - when I first got started with CSS, I made a point of building a good, fully commented base stylesheet. That stylesheet has changed many times and remains a terrific resource.
However, when I ran into my own box model problems, I started using 'Mo Pi'. As in, "I'm not fat enough, I need to eat mo pi." Strange, but it worked for me. Of course, I put on twenty pounds while learning CSS...;-)