For running the first episode of Savage Tide (which is similar to a lot of wotc adventures) I:
1) Read the adventure in it's entirety.
2) Read it again.
3) Skimmed the statblocks, highlighting any special abilities I should keep in mind, Damage Reductions, and the like. I also put some thought into how the PCs would react in the encounter.
4) Took a nice long look at the maps. When you skim the map while reading room descriptions, you don't really get an idea of what's going to happen. Where are the chokepoints? Is it a linear map, or what? The dungeon I was looking at was an exploration map that turned linear - I didn't figure this out until I really looked at the maps.
5) Think about the maps. One thing I didn't notice about the adventure was that there are numerous fights in very cramped conditions. When I actually started running it, there were a few times where all the combatants couldn't even fit into the room they were fighting in! I had to think on my toes because I didn't see the problem coming up.
All this work takes about two, three hours... longer for a bigger adventure. Of course, while steps 1 & 2 are important, you could only do the next steps for what you think you're going to run that session - in fact, it's probably a better way to go.