Diligence: You must practice regularly. The way to improve performance under stress is through constant drill and rehearsal. When it's for real, you fall back on what you have learned instead of succumbing to panic. At the U.S. Army Sniper School, the conventional wisdom is that you expend 5,000 rounds in practice for every round you fire at the bad guys.
Cold Blood: This means you are not undone by pressure. In hunting it also refers to a willingness to kill. I think a good many cases of buck fever are caused by a reluctance to take life. Good shots do it impersonally; they don't relish it, but they don't shy from it, either. If you are able to kill with complete indifference, perhaps you should take up another sport.
Faith: That is, faith in your rifle—and the way to become a true believer is to use a gun you shoot really well. Yet we often go astray. We read about which cartridges will do what, at which ranges. We obsess about making shots far beyond our practical limit. Then we go out and buy veritable cannons whose recoil and muzzle blast keep us from shooting as well as we can.
Experience: A friend of mine who has been hunting for six decades and has the skills to show for it said that after you've taken 300 head of game or so, you start to calm down and figure out what you can and can't do. This is one part I can't help you with, but I hope you have fun figuring it out.
Will: I've known at least two shooters who simply willed themselves into a state of excellence. A couple of weeks ago, I was shooting against one of them at a contest in which a perfect score is 50, and there are very few of those. I had a 48. So my competition sat down and stared at the ground awhile, thinking I know not what, but willing himself to beat me. And he did.