I start with what the story calls for, figure out the Main Event for the scenario (i.e., "rescue the princess," "encounter the beholder," or "the hallway of infinite gelatinous cubes") and put that in place, then flesh out the rest of the map with more or less random encounters that fit the overall theme. (Thus, if it's a sewer, I'll drop in rats, oozes, crocodiles, etc., or if its a trek up the side of a mountain I'll put in displacer beasts, wyverns, and owlbears as appropriate.)
Certain recurring motifs I use a lot are:
* Displacer beasts, hook horrors, and orcs are my favorite monsters, apparently, with minotaurs, bugbears, and beholders getting an honorable mention
* Arcane experiments generally go horribly wrong, leaving dungeons full of dangerous encounters and peculiar situations -- and dead wizards
* Often there's an encounter that's way beyond the abilities of the group, but which can be fairly easily avoided or escaped if the players keep their heads and don't go rushing in
* Usually, all or almost all of a single "dungeon" (even if it's multiple levels) will fit on a single sheet of 5x5 graph paper; any given scenario may have from one to four such "dungeons," depending on the complexity of the scenario.
* Traps are rare, but when they appear, they tend to be vicious
-The Gneech
