Mike Rundle has a handy overview of some design principles: How C.R.A.P. is Your Site Design? His acronym stands for Contrast, Repetition, Alignment, and Proximity. Four good principles to keep in mind when designing pages.

My rule of thumb has always been: use as few things as possible (faces, sizes, colors, widths, and so on). This inadvertently lead me to Repetition and Alignment, but I think makes my pages a little plain.