You've got to get into "kill" mode. Don't look at the antlers. Just focus on the spot you want your arrow to hit. Think about when you are going to draw, and where you are going to aim.

I used to get buck fever really bad (even on does!), to the point where I don't know how I even made the shot. Experience helps more than anything. Now, that I've got 8 bowkills under my belt in three years, I'm getting much more "calm". I still get excited, but it doesn't interfere with my ability to make a good shot.