Props.
I have the controller propped up against something so 'L1' is held down. Then I prop something on top of 'O' with a turbo-fire controller (works even better if you have auto-fire, since you wouldn't have to prop anything on 'O'). Finally, I prop something on top of 'LEFT' so Cloud will constantly go in circles to the left. During battles, he'll attack the insant he gets a turn, thanks to the Fury Ring, and he'll attack all enemies thanks to Slash-All. If he's the fastest character of the group, then he'll most likely get the first turn, and if he's not the fastest, then one or two of the enemies may be taken out by the other characters because of 'O' being constantly pressed. If you're fighting weak enemies, you could probably go a few hours without having to even look at the screen.
You need 'O' to be constantly pressed on turbo to skip the spoils screens (where you get your EXP, Gil, items, and AP). 'X' might work, to prevent other members of your party from attacking, but that might cause you to not get any items (which may not be a big deal with you, but I just have to rob the enemies of all their stuff after killing them).