Breathe. Breathe deeply. Lower the gun briefly if necessary.

Don't shoot if you're wobbling around. You might get it but you might just wound it and that's not ethical. The self-control required to pass on a bad shot is very related to the self-control you need to not get buck fever and wobble around.

Practice. This comes from shooting out of season so that you don't have to think about what to do. It also comes (in my experience) from having had more opportunities to shoot deer. It gets better with time, but not time alone. So practice, breathe, and be willing to pass on the shot.