The two absolute biggest hitters are:
1) The crush. If you're not sure if you've been crushing enough, then crush it twice. Your efficiency will probably shoot up by 10 points overnight.
2) Volume measurements. Need to be as accurate as humanly possible. If you were supposed to end up with 5.0 gallons but actually ended up with 5.4 gallons or 5.6 gallons or something else, your efficiency will be WAY off.
Make sure you're following those two things, and your efficiency will skyrocket. There are other possible effects, but none as huge as those two. Well, okay, I suppose there is a third item, but it should be a no-brainer:
3) Good to the last drop. If you didn't get all your wort out of the mash tun, your efficiency will be impacted. If you are a batch sparger, the first runnings is more important than the sparge. Get it all out. You could see an increase of ~5% from doing this.
Okay, now that's all I'm going to say for now.