I don't know about you but I can say "I'm going to shoot a clean match" or "I'm not going to shoot any penalties" and get the same results. I'm not sure if my head is built differently than most (yeah yeah I know there are going to be 100 jokes for that one), but saying "don't" in a goal doesn't get me to focus on what I don't want to do, it's merely a way for me to remind myself to focus on the present and on what I'm doing.
It doesn't matter how many things you state in a negative way...If you don't want to hit that no-shoot target, you won't - unless you let yourself get out of the present. The reason those types of "mantras" (for lack of a better term) hold people back is because they are thinking about that shot before and after it happens rather than just being in the moment and letting their vision guide their shooting.
In that same manner of thought, by saying you don't want to do something in a stage it makes you concentrate on it - which is an action from the conscious mind. If you have the fortitude to stay focused in spite of that, nothing you say will ever really have an effect on your shooting. In actuality, saying you don't want to do something in a stage is a great test to see if you will handle pressure effectively. At least it is for me.
Concentration = Conscious
Focus = Sub-Conscious