The first thing I try to do is get my breath and control my breathing. Seriously, except on those occasions where the bird showed up so fast that I didn't have time to react, I start to shake, my heart pounds and I can't get my breath so the first thing I have to do is calm down and make sure I a in the right state of mind for the shot. Given that little issue, Bobgobble2 pretty much has the rest of it down. The only thing I would add is that you must be prepared to react to what the bird does. No matter how much I plan and think I have it figured out, 50% of the time the bird comes in from somewhere I didn't expect (maybe my hearing is not that good) and I have to be ready to re-adjust.