Make sure you have all the roles filled: tank, healer, nuker, and thief. You can combine two in one (such as a mage/thief, for example), but you'll want these roles covered by your party.
It's a good idea to try to min/max a bit. So build your characters with high scores in their key abilities. For example, for fighters, you ideally want 18/00 STR, but anything over 18/75 is fine. At least 16 CON.
Make sure you have ranged weapons and use your spells to turn the tide in your favor. Spells that disable (paralyze) are good to keep at least one mob out of the action while you tend to others. Charm is great to turn a mob against its allies.
Equip your tanks with the best gear you can find/buy. They'll be up front and will take the aggro almost all the time.
Pause often and give orders. Make sure all your party members are doing something useful at all times during the battle. Set up their AI routines as well, so they'll tend to do something useful on their own. Don't be afraid to use your resources (potions, wands, scrolls). Everything helps and can make a great deal of difference during fights. Set the game to pause when an enemy is sighted. This will ensure you don't waltz right into the middle of a huge group of mobs and get yourself wiped.
Tip: after each battle, check your ranged members' ammo supplies. Restock their quivers after each battle. It's a real drag to have them run out of ammo and go running into the fray with a melee weapon or bare hands when they're soft and squishy.
