Lots of good ones in this thread, especially muzzle control and keeping the finger off the trigger. I want to add two more:
1. Engage the safety or decock after firing!!! I'm talking about the newbie with a semi-auto pistol who fires 3 or 4 shots at a target and then pauses to see where he's hitting, while holding the gun down by his side cocked and finger on the trigger. Seen it many times when teaching new people.
My main one, though:
2. Get over your fear of the gun. I see many newbies who pull the trigger slowly, taking 30+ seconds to fire a shot, as if a nuclear bomb will go off when the hammer falls. They grimace, clench their teeth, squint, twist their heads back, etc. When it finally fires, they stand there with that deer-in-the-headlights expression.
I explain this to them. One exercise I like to do with new people is have them stand there and empty 2 or 3 clips, just rapid fire, one shot after another, and do it with a COMPLETELY relaxed, perfectly straight face. No lip-twitching, no blinking, no nothing. Just hold it up and go bang bang bang, appearing to be utterly bored to death. This really helps a lot of people.