One of the DM's creeds is the same as the Boy Scouts: BE PREPARED.
Without going into all the details let me just say that nothing irks me more as a PC as when the DM is looking something up that he clearly should have been prepared for. Make lots of crib sheets for yourself. For example, the Detect Magic crib sheet is laminated and taped on the table right in front of me. Be familiar with your NPCs, have a battle plan worked up for 5-10 rounds into an encounter (w/ contingencies - damn PCs are always doing something you'd NEVER expect them too....little bastards

). Go through their attack regiments in your head - make them YOUR guys (don't get too attached though.....

)
A good grasp of the rules never hurts, but without instilling confidence in your players that you are prepared for a session you will quickly lose their respect in that session. Be confident without being a bully, don't harp too long on any adjudication - these are best resolved out of game.
More to the point of your question: Read the DMG, keep it in the bathroom and randomly open up a section and read through it. I am always finding new stuff in there.
Whoever suggested using tabs in the DMG was right on! Kudos, Woas. I use the index tabs in both the DMG and even moreso in the PHB. They're self-adhesive, durable, and the sticky portions are clear (and rarely cover any relevant text anyway due to their slim design). They have made accessing information much, much easier.
Above all though, I say BE PREPARED!