My group (as chronicled in our story hour) is 24th to 25th level with 6 characters, 3 cohorts, 1 mount, 3 familiars and 2 animal companions. I feel your pain.
First and foremost: Rodrigo tells it true. Don't Sweat the Small Stuff. As he clarified, that's not '
ignore the rule', it's '
don't get bent out of shape if you forgot that Bless was running during the last round'.
At high levels, you can have literally dozens of spell effects in play at one time, plus magic items, plus class special abilities and effects from situations, locations and monsters, too. It can be a mess, if you let it.
Trick 1:
Player-specific Math. Let the players sweat their own numbers. You're already responsible for all the NPCs and monsters. You don't need to factor in their specifics, too. Advise them of certain details, and let them sort out the math.
Trick 2:
Notecards. Have an item or spell that will modify more than yourself? Write it on a 3x5 card, with a summary of the bonuses, and put it on the table. Train everyone to check them during combat. "
Oh, cool, you've got Holy Aura going, and I'm 10 feet from you."
Trick 3:
Pre-compute the hard numbers. Have players roll their attacks and damage ahead of time, if they know that they'll be attacking, and have them compute all the numbers so that when you get to them, results should be quick. If players aren't prepared to do that much, at least have them know exactly what they're bonuses/modifiers will be when the rolls come. All Spellcasters, as a rule, should know the DC of the spell they are casting BEFORE you ask them what spell they're going to cast.
Trick 4:
Quick Adjudication. When a rules dispute comes up, set a time limit (we usually use 5 minutes, tops) on making a ruling. Assign players the task of finding relevant passages to argue their case. Hear quick evidence, decide what you think is correct and make the call. If it's a matter of player life-and-death, you may opt to let them take some more time...but don't let the game be derailed in a rules discussion. Don't harp on the numbers.
Trick 5:
Check before Worrying. Sometimes a player will not be entirely sure of all the numbers when his turn comes. You, as DM, probably have a better idea of his chances, whether it be to hit, make a save or contest a skill. If the halfling wizard wants to grapple a 20th level Ogre fighter, have him roll FIRST, and see if it's even worth debating the specific. If he rolls a 20 and the ogre rolls a 1, do the math. If the Ogre rolls a 19, and it's obvious that the wizard doesn't stand a chance of winning, stop there and move on to the next action.
Trick 6:
Stay Stock. Monster stat-blocks getting you down? Use more standardized beasties, and settle for simple advancement, core classes or just double their hit dice. Do what Piratecat has often done: taken a standard monster and change it's appearance or description, swap a power here or there, and then send it against the party. A white slaad might be familiar, but change it to a mercury-like liquid shape and give it some extra hp, and players may never know the difference. Less number work for you, more fun for them.
Trick 7:
Fudge it. Sometimes you just need to stop worrying about being 100% rules-correct every time. Approximate if it isn't going to be subject to intense scrutiny. Take a purple worm, double his hit points, increase his BAB by 12 points and let fly. The players will neither know or guess if you remembered to give him four extra feats and the right amount of skill points.
You get the idea.
Our group sometimes has multiple PCs around the table, but mostly just for quick SRD lookup, in my case. Some of my players have devised excel spreadsheets to compute their attack scores for any situation: the archer can tell you, in seconds, what her attack numbers will be at 1000 feet when doing a manyshot with adamantine fire arrows into melee in a windstorm. Others just check their sheets.
