I prep my DM's "grab bag" much more than the adventure. In my grab bag are character ideas, monster/evil boss ideas, interesting places, unique tricks and traps, cool treasures, and plot ideas. Oh, and a list of suitable NPC names and place names.
Along with all those, I have pictures of anything and everything interesting that I might use to show players--usually they are monster pics or treasure, but I have NPC pics, and eventually maps that I show them too.
Another section of my grab bag is nothing but maps...I have a pile of maps. Need a druid's grove? Got it. Need a small town? Got it. Need a catacombs? Got that too. There is no such thing as having too many maps.
Having firm ideas of what you want the locals to be like are important too. Ideas about trade, economy, politics, laws, and whatever locals are likely to be troubled by the most these days, are also needed. But those kinds of things are easier to be flexible with from place to place unless you run a more political campaign.
Having all of the above, I often spend almost NO time to prep a night's adventure (and my DM sessions run 8-10 hrs at a time). I do sketch out a mindmap outline sometimes for more sophisticated plots that involve more than two or three important NPCs, but it's just that, an outline to help me keep everything in perspective. And, finally, I do keep my eyes peeled for more new stuff for the grab bag--that's just a constant.