Scrivener of Doom
Adventurer
(snipped lots of good stuff) A certain amount of this "low prep" has to do with the few times I have done elaborate prep (set up traps, made special monsters, etc.), it's mostly been in vain. The players find the trap, they defeat the special monsters before it has a chance to use more than one of its special abilities, and basically make the prep-to-utility ratio very bad. Rolling with the punches I find to be a more rewarding and fun gaming experience for both me and my group. (snip)
I think one of the keys to running 3.5E successfully - which means with minimal DM burnout - is to understand that extensive preparation of stat blocks, in particular, is going to be wasted. Keep it simple and keep it moving.
What I also find helpful in terms of the seat-of-the-pants style you describe is having, in the back of your mind at least, the rough outlines of three or so old school location-based adventures that can be easily adapted on the fly when you absolutely need something more substantial to run.
One of those for me - and, no doubt, many others - is the Caves of Chaos from B2 Keep on the Borderlands. The basic conceit of a series of caves (or they could be dwarven mines or minotaur tunnels - whatever) inhabited by multiple tribes with a mysterious temple (or arcane oracle or archetypal evil wizard) acting as some sort of locus of loose control is a location for which there are nearly infinite variations on the basic them but which can be dropped in almost anywhere.
Almost every campaign I run includes a location like this and it's my "go to" location when I am otherwise scratching for ideas and the players really want to explore and fight.
There are other adventures like that - Raiders of Galath's Roost from Dungeon 87 (?) is also like that for me - which are fairly simple in concept, have maps you can easily change on the fly and using the adventure's basic framework you can quickly improvise a decent adventure with next to no preparation.