As pointed above, any extra damage can make lethal non lethal attacks
Also, the basic thing, non lethal works only with melee. Magic and ranged are always lethal
Elemental damage can also kill with non lethal
Conditions that add extra damage (ex: bleeding) will also kill after non lethal turn unsconcious
Basically with the equipment and abilities you have usually in Act III, the safest way to make non lethal is reduce the enemy HP. Once it's low, make sure he doesn't have any harmful condition, unequip your weapons and use basic fist attacks (make sure you don't have also any "more unarmed damage equipment")
