Yea I agree preparing ahead of time and writing stat sheets up or typing it up on the computer if you use the pc at the table helps a ton.
Another thing is when you have large groups of baddies that can have various feats and skills (from classes for instance) try to make all the 'foot soldier' types witht he same feats and skills and only the leaders having different feats/skills and/or classes. This is always good when say you have 5 or more baddies with a leader type. Makes it easier to see and remember what feats/skills the 'fodder' has and allowing you to concentrate on the 'boss' baddies abilities. Less chance of getting abilities mixed up or to simply forget who has what.
However, once in awhile make a large group of baddies who are say all fighter types with a spellcasting leader or two, yet give them all different combat type feats and really prep before hand. Make sure you understand each baddies abilites and write it up in a way that is easy for oyu to 'refresh' with during the encounter at the table following the advice everyone has given so far.
This will make sure the players dont expect the 'fodder' to always be the same, and concentrate on the leaders only. They will always spend those first few rounds trying to fathom the enemies abilites.
Even when using the cookie-cutter approach you can mix it up by designating ahead of time that 'fooder' baddies #1-2 will tend to use seach and such feats/skills while 'fodder' #3-4 will try and use these feats/skills. So even though the majority of the baddies in a single encounter are basically clones, the fact that their not all using the same feats/skills/tactics will keep the players on their toes and they might not even ever realize you just threw a ton of clone 'fodder' at them.
The best advice given by the others so far though is doing up the stat sheets and making sure you list their feats and understand what each feat or feat combo can do in different situations.
Highliting or Bolding feats and special racial/class traits is something I do on my stat sheets, and listing what the feats/ability does in a minimal fashion, so I can quickly glance at the stat sheet and get back to things fast. Give each feat/ability its own line on the stat sheet and bold/highlite just the name and then in normal text list the mods/bonus/ability in the shortest form that wil help jar your memory. By using the cookie-cutter approach you also limit the number of stat sheets oyu have to dig through and can have a entry or two near the bottom saying what feats/abilities/tactics #'s 1-3, 4-6 and so on will concentrate on trying to use. And if a certain way seems to work better against the group and your feeling particulary DM-ish you can have the other baddies change to the same tactics and watch your players sweat it out
