I have a Jedi character in a Star Wars Saga edition game, who does not want to kill without good reason. If he uses a force power like Force Push, it causes a set amount of damage which usually kills the opponent. Is there anything I can do to prevent this?
Instead of Force Push, you could use:
Force Thrust: You use the force to push a target away from you. (Saga Edition Core Rule Book, p. 98)
There is a nice little Non-Lethal Force Powers Index that should be able to help you out.
As far as using lethal force powers and not dealing damage, or reduced damage, that would entirely rest upon your GM's shoulders. There isn't a mechanic available that allows for such a thing (to the best of my knowledge). If I were GM, I would probably allow Force Points to vary your damage dealt, rather than increasing your d20 roll, depending on what your intentions were. Speak with your GM about this, he will in fact be the arbiter of such a thing.
There isn't a way to augment force powers to deal less damage by RAW; the only way to deal less damage would be get your GM to implement a houserule. There are many force powers that don't deal damage and instead move the opponent down the condition track, which is the best route to go for force powers if you don't like killing enemies and can be vastly more efficient than just augmenting a force power to deal less damage if you have an amazing use the force check.
I don't have a reference handy, but the devs clarified years ago that force powers can always be 'dialed down' to avoid killing someone with them.