I’d like to share a classic DM’s tip that really helped me in the last game I ran.
At the beginning of the session, ask your players for their core stats: AC, Fort, Ref, Will, and the Perception (4E) or Spot/Listen (3E) skill bonus. Write these down in a table with player names on the left and stats along the top.
This table has two uses. First, you can see stats at a glance, so you can quickly tell if your attacks hit without asking the player. This is particularly useful in internet games where communication is slow, and in 4E games where Fort, Ref and Will are flat numbers like AC instead of player rolls. Remember that your players may have temporary bonuses or penalties that aren’t listed on your grid, so take care if your roll is only a few points off the number when you know that there are modifiers in play.
Second, you can use it to make secret rolls for the players. It’s particularly useful in third edition for secret Spot/Listen checks (so you don’t tip the players off if no characters notice anything) and secret attack rolls and saves (mainly for illusions and mental domination effects). You might also record other skills where the DM rolls so the player doesn’t know if he’s failed, such as Stealth (4E) or Disable Device (3E).
If it helps, you can do the same for your monsters. I find this useful when holding the Monster Manual open at several pages and flicking between the two. This keeps important stats like a monster’s AC readily available during player turns.