You can try having an assistant DM who might help tracking the monsters (as in, take care of 2/3's of em (at least), esp. the BBEG's with many options, so you can concentrate on tracking circumstances and do the other monsters.
I've noticed that a party twice the size is
more than twice as powerful, since they're more likely to have all the useful stuff covered, and since things like supporting aura's and so on just become so much more powerful. Conversely, they'll have fewer hitpoints each, and as many have noted, there's a much greater chance they'll die each combat.
You could even try splitting the group into two, perhaps?
In any case, the best thing to do is to use a bunch of monsters (say, 3-4 times as many as normal). You might also tweak monsters (and even player characters, maybe) by giving them all more HP, AC, and saves, to stretch out their life expectancy without improving offense. In principle, the basic idea is that a stable, not-too-lethal combat for 4 people becomes far more lethal as you increase the numbers, and you therefore compensate by scaling back offense in relation to defense.
A large group will also disturb the magic vs. physical balance. In large groups, area effects or even just multiple target spells (say, mass resist energy in the spell compendium?) become far far more powerful, but a fighter is still a fighter (and a rogue is still just a rogue). To make matters worse, in general, most monsters aren't huge area-effect slingers, and don't scale as well as spellcasters, and even if they were, they'd not be nearly as effective simply because one DM can't control them as smartly as 11 individuals that know their characters strengths inside out. How can you fix this? Well, you'll need to increase the effectiveness of fighter-types, esp. melee one-vs.-one types a lot. One simple improvement is to allow max HP for everyone - this helps fighting types more than lower-hitdie magic users. You might also want to make class-by-class modifications to improve classes such as the rogue and fighter. You can also make everyone play a certain race or template which includes a level adjustment and/or a racial HD. This generally hurts fighters and skill users less than magic users. (i.e. consider a party of bugbears or whatnot)
Another thing to be careful about, which is potentially even more annoying than magic users, is aura's. If your party contains a bard, a paladin, a marshal, a dragon shaman, etc, combat will become extremely complex since the radius of these auras will be frequently less than the scale of the battle meaning that each attack and effect becomes a game to determine which aura has effect now. This is exacerbated by the ease with which these effects can change (generally constantly). If you add a cleric using mass buffs, an arcane caster with haste, a druid with mass resist energy and whatnot, then pretty soon an individual creatures (PC and NPC alike) stats constantly change and it's extremely difficult to manage everything. You might simply want to ban especially problematic classes, or not allow them to change their aura in combat (which is an entirely reasonable limitation, considering that such a large group will benefit more than a small group
anyhow).
Summary
- Maximize hitpoints (and improve other survivability stats at the expense of offensive abilities)
- Consider helping low-magic characters
- Consider banning or simplifying complex administration heavy abilities such as aura's and certain spells
- Consider tilting the balance by using races with LA and HD to avoid the worst excesses of magic users.
- Apply these ideas to PC's and NPC's
- Use a lot of monsters as opposed to a few strong ones - roughly as many as there are PC's, on average
- Consider asking for a second DM to help you run the game.
