I've found that the best time to work on dungeon creation is during the world-building stage. Is there going to be a lot of dungeon bashing in the regular game? Yes? Then deciding on who built all these dungeons and ruins is the first step. Ancient civilizations, elder races, a prior high-tech era, overactive dwarves...someone built heaps of stuff, that is now overgrown, ruined and lost.

As for dungeons, have a few set pieces, a few drop anywhere encounters, and a robust random encounter table...and don't be afraid to have multiple encounters coincide if it is warranted. 99-100 on most of my charts is "roll twice, then roll d6, 1 the two meet and are friendly, 2-3 the two meet and are neutral, 4-6 the two meet and are hostile". Adds a bit more dynamic to the dungeon...