In my previous employment I created dozens of templates for websites.
The most useful too I ever discovered is the Firefox Web Developer Toolbar.
It has a wealth of small useful tools. My favorite feature is the ability to edit the CSS and see the results in real-time. This saves on the whole edit - upload - refresh cycle. Watch out for ie CSS inconsistencies though!
Off the top of my head, these are the most important gotchas.
double margin bug [google: double margin bug]
incorrect (but more intuitive) box model [google: box model]
incorrect (but more intuitive) float clearing [google: clearfix]
FireBug is another really usefull Firefox plugin for more in depth analysis.
