Contrast
"To be effective, contrast must be strong--don't be a wimp.  If two elements, such as type, rules, graphics, color, texture, etc., are not the same, make them very different--don't make them almost the same!"

Alignment
". . .simply means that items on the page are lined up with each other.  Lack of alignment is the single most prevalent problem on web pages.   It's a big problem on printed pages as well, but it seems to be even more ubiquitous and disastrous on web pages."

Repetition
"Each page on a web site should look like it all belongs to the same web site, the same company, the same concept."

Proximity
"When two items are close, they appear to have a relationship, to belong together.  When items are physically far from each other, they don't have a relationship.  Often on web pages (as well as on printed pages) many items are orphaned unneccessarily, and many other items have inappropriate relationships."