Good question Cat, and a tricky one to answer! I'm no expert at CSS either, but here are some suggestions on how to improve your skills:
1)
Forget about IE6. It will just hurt your brain, limit your knowledge of newer CSS, and make you use all sorts of nasty hacks. If you must make a site work with IE6, get it working with FF/Safari/Chrome/IE8 first, then add IE6 hacks to an IE6-specific style sheet by using conditional comments:
http://www.elated.com/articles/internet-explorer-conditional-comments/2)
Learn all the selectors thoroughly. Once you understand how to select practically any element in the markup, things get easier. See
http://www.w3.org/TR/CSS2/selector.html3)
Use good reference material. http://www.alistapart.com/ has some great articles on various CSS tricks such as layouts and sprites.
http://www.css3.info/ is great for learning about all the new cool stuff in CSS3.
http://www.csszengarden.com/ is also good. I also reviewed a CSS book recently that does a good job of explaining CSS 2.1 in detail:
http://www.elated.com/articles/book-review-getting-started-with-css/4)
Lots of practice! The more CSS sites you build, and the more you challenge yourself to implement complex designs from PSDs, the better you'll get and the more tricks you'll learn.
Hope that helps!
Matt
--
Matt Doyle, Elated
Second Edition of my jQuery Mobile book out now! Learn to build mobile web apps. Free sample chapter: http://store.elated.com/