What's your GMing style?
I'm more of a 'deal with it as it comes' type of GM. I'll write up the major NPCs (BBEG, town mayor, so on) and their motives/goals. I then come up with the general setting and how NPCs in the area would act and react. After that I let the PCs come in and go with what they want. Often times this means coming up with new NPCs on the fly, but that's pretty fun to do.
Any of the NPCs plots and plans will remain in motion even if the PCs are ignoring them. So if there was some Evil Necromancer trying to summon a giant horde of undead they'll work at that and if not stopped, get it done. The PCs will then have a larger issue to deal with, but it also gives more openings to how they will, when they will, and if they'll even be against it (if not join up with the BBEG).
I find that I'm pretty bad at riddles and puzzles found in dungeons. Because of my methods I don't spend much time writing up future encounters leading to the more complex types of encounters being tougher for me to come up with.