Regardless of what kind of Web development you are doing, you will always need to work with CSS before launching your site. I've been reflecting on best practices in CSS lately. So here is my list of best practices. This is not by any means a complete list or the gold standard for CSS development. This is just what I have learned so far (although I doubt many would find any of these statements controversial). Feel free to add your ideas in the comments.

1. Start with 100% valid semantic xhtml. Make your markup describe the page or template you are creating. Invalid markup will create problems, even if one browser is forgiving of invalid markup, another will not likely be as forgiving. Many text editors provide this feature. If nothing else, use the W3C's validator.
2. Reset it. Use something simple like * { margin:0; padding: 0; } to help assure all browser are interpreting your markup the same way. You may even consider using Yahoo's CSS reset YUI library.
3. Use Firebug: Firebug has revolutionized CSS and Javascript development. Firefox's Firebug lets you click on any element on the page and see exactly which CSS rules apply. It even lets you change, add and remove CSS rules on the fly to test various possibilities.
4. Use a CSS aware editor. I've started using MacRabbit's CSSEdit this year. The autocomplete as you type feature alone was worth the $40.
5. Get everything perfect in one browser (probably Firefox) first and then test in other browsers.
6. Set up an environment where you can test all major browsers quickly across platforms. For me this means running Parallels, Windows XP, and Ubuntu on my Mac. Windows users need to test on the Mac as well (or at the very least test on Safari for Windows). I recently shelled out hard-earned cash for for XP and Parallels - but if you are serious about CSS development, this is necessary.
7. Use well established techniques like unordered lists for menus or well known equal height columns techniques. Why reinvent the wheel?
8. Be aware and beware IE idiosyncrasies. IE usually causes the most headaches. Know about hasLayout, the peekaboo bug and the like. If you must, feed IE its own stylesheet via conditional comments.
9. Use a code repository to allow rollback to an earlier version if you make a mistake.

These things have helped me in CSS development tremendously.