If you don't have an eye you shouldn't be doing this stuff except under duress. Asking for rules is asking for an algorithm that would let you "design" (ok, lay-out) mindlessly and sightlessly. Mostly, people don't want to behold those results, and editors don't want to reformat every document. Of course there are guidelines, best practices, a history of what's known to work/look cool and of what's known to not work/suck/look crummy.