Two simple tactics, both relatively-simple to implement from the GM-side, both of which I regularly use in an attempt to curb the kill-count:

1. Most systems have a provision for characters taking incapacitating-but-not-lethal damage. (Unconscious at 0 hit points, for instance.) Apply this to opponents. It's an application of some of the other answers' "give the players an alternative once weapons are drawn" advice, with the merit of using mechanics with which the players are completely familiar. (And are grateful for themselves, and thus find fair.)
2. If an encounter includes intelligent creatures or the pet-like companions of intelligent creatures then they've all got names. Come up with (or generate) them and use them. It's an application of other answers' "humanize the opponents" advice.

Neither are big effects, but I find each puts another feather on the less-killing side of the scale.

Two simple tactics, both relatively-simple to implement from the GM-side, both of which I regularly use in an attempt to curb the kill-count:

1. Most systems have a provision for characters taking incapacitating-but-not-lethal damage. (Unconscious at 0 hit points, for instance.) Apply this to opponents. It's an application of some of the other answers' "give the players an alternative once weapons are drawn" advice, with the merit of using mechanics with which the players are completely familiar. (And are grateful for themselves, and thus find fair.)
2. If an encounter includes intelligent creatures or the pet-like companions of intelligent creatures then they've all got names. Come up with (or generate) them and use them. It's an application of other answers' "humanize the opponents" advice.

Neither are big effects, but I find each puts another feather on the less-killing side of the scale.

Quote from tonight's game:

PLAYER1: I'm going to shoot... oh, hmm....

PLAYER2: What? Just shoot the guy near the building! He's busy loading his crossbow.

P1: That's Karsten. He's got a name. I kinda feel bad shooting him.