*I'll add this just above since it's much shorter - if you have a combat that you feel is 'well in hand' (for the PCs, likely not appreciated the other way). And you decide it's just a matter of time. And the risk of the enemy getting very lucky the last few rounds is outweighed by brevity in your mind (perhaps the real challenge is to come). I've seen where the GM takes over and describes the 'mop up' rounds. Dramatically and where the PCs still feel proud of what their characters did to finish things. But in a paragraph rather than days of rolls.

Similarly- not every opponent would or does fight to the death. A foe in bad shape or the last one left from a group could drop their weapons, fall to their knees, and beg for mercy. Perhaps in pathetic desparation, being realistic, sincere 'repentence', or with plans of sooner or later treachery. Changes from combat to PC decisions to make. The group and you as GM may very well be fine with instant execution for many reasons. But the naive foe need not know that, yet. (They could also just try and run away.)

Hero Points (Pathfinder). There may be something similar in 3.5 or you could just pull them over for your campaign. Get one per level, use them and they are gone. Can be used ahead of a roll for a +8 or after for a +4. To reroll. Take an extra standard action. Cheat Death. Etc. If a battle is being exceptionally difficult - allows for 'these heroes have some extra mojo / divine luck / whathaveyou'. A GM in a Pathfinder PbP game I'm in pulled them in after we had some battles vs elite enemies go for quite a while. They have been good. It's a flavorful as well as functional thing, well described below. You certainly can't use them every encounter, but they help. (Not just on an attack roll. Passing instead of failing a critical save vs an effect that would really hinder a character has helped as well as for attack rolls.)