After trying virtually every method under the sun in order to improve my DM'ing, and consequently having numerous disaster sessions, I'm now thoroughly entrenched in the "Wing it!" camp.
I think that, especially with 3rd ed., it's the best way to go because it frees up both your mind and your time. You can much more readily react and adapt to player mood and in-character actions. The mere fact that you aren't spending time referencing things whenever a PC goes into room #223a, means that there is a much more fluid feeling to the game and the players will also feel that their actions aren't entirely futile since what they do can affect the outcome to a much greater degree than if you are tied to a plot line.
Of course, reading through modules never hurts and can spur many ideas and can be called on in a pinch. But using them by wrote is definitely a bad option, IMO.
And all this is not to say there can't be a theme to your games or an overall goal. I just think that DM'ing on the fly allows for much more creativity, imagination and life to a game than does simply going by the book.
Steal their maps, their ideas, their characters, traps and treasures for sure, but ultimately I think you and your players will enjoy the game MUCH more if you just go with the flow and see what happens.