I only detail major events and let the rest of the pieces fall as they may. I do like to have a good outline and some generic back-up encounters to cover some of those odd eventualities that players always seem to stumble upon. I also spent far more time detailing things in the past. I really hate wasting my time going into great detail on something, only to have a player figure out some bizarre twist to completely avoid it. So, to avoid my desire to railroad them into that awesome 10 pages of detail, I wing it much more. One good thing for me, I usually use the same adventure on 2 or 3 groups making what preparation I do worthwhile.