D&D has a lot of rules!

[Henry]

Similar to Baumi, when I run 4E, I have the DMG open to pages 184 and 185 (or rather, I have their charts pinned up on my DM screen) and I almost entirely ditch the monster manual. It works well enough for me that I can run most monsters straight from those two pages, and fudge their powers off of the damage charts and a short list of special powers I keep in my head.

 

[Celebrim]

When I was playing 3e, I cut down on in-game referencing by doing out-of-game preparation.

This. Whenever I flip open a book during a session, I feel like a failure as a DM.

It's even more important to have the stats in front of you so that you won't have to open a book, than to get things exactly right. If the DC of a power is off by 1, that's not a big deal. If you stop play for 5 minutes to look up the DC, that's huge.

When in doubt, use your instincts as to what is fair, and keep the flow of the game moving.