the simple things
We "all know" that using relative units of size is a better idea than fixed units, right? Well, sort of. Browsers should resize whatever they're given according to user preferences, so it shouldn't matter. But they don't, so it does, so to speak.
So what do you do if you want to use EMs but you're used to pixels? You refer to the Approximate Conversion from Points to Pixels chart over at Reed Design. The interesting thing to note is that you can use EMs for everything, including widths, heights, etc.
A nifty trick is to start out by setting a percentage font size on the body, then use EMs for everything else. It avoids having to use decimals for smaller text.