Paralysis by analysis.
To avoid pulling a Dr. Phil (catchphrase without anything productive), I'll attempt to be helpful.
Touching on Aimee's comments, there's no doubt that self talk can influence performance. Always speak to yourself positively, focusing on things that you want to happen. I.e., don't say "don't bend your arms", say "elbows straight"; or instead of "I'm not sure I can get this", say "I've done this weight before". Etc.
A consistent pre-lift routine will help bring your mental focus. Sports like tennis, golf, shooting, and weightlifting are all very conducive to developing a pre-performance routine. This might be as simple as checking your shoelaces, taking a tight grip, 2 deep breaths, and dropping the hips into the starting position. Or you could talk to yourself while spinning a basketball like Karl Malone used to before shooting a free throw. Whatever works. The key is to be consistant. Take the same time between elements every time, same breathing, same mental cues, etc.
Visual imagery would be an appropriate part of the routine: picture yourself-in real time-going through the lift (successfully). Try to be as detailed as possible in your imagery, feel as if you are actually performing it.
Lastly, the use of cues is very useful. Pick one or two very simple words or phrases that focus on important aspects of the lift. Like "1...2...3...Jump!" or even simply "explode!". Don't try to talk your way through the entire thing, just focus on those one or two cues.
If all else fails, what usually works for me is "Oh my God! Just pick the @#&ing thing up!"