Pre-arrival preparation is important. Make sure you have everything with you that you will need or want when you are shooting (gun, choke tubes, plenty of shells, well marked if they are different, glasses, vest and/or shell bag, towel, gloves, hearing protection, spare car keys, sugarless drink, a Bandaid or two and anything else that you even suspect you "might" need - a spare cap, pair of shoes, shirt and pair of socks, maybe. (What if you break your shooting glasses or lose your hearing protection? (There are a lot of "what-ifs" - consider them. Is there a price to pay for being over prepared?)
The key, in my opinion, is to reduce stress as much as possible; stress causes muscles to tighten, which can affect swings, causes fatigue to set in early and uncontrolable conditions of wind and weather to bother you more than they need to. Remember: Everyone else is dealing with the same conditions. If they aren't by shooting earlier or later, think how proud you will feel by overcoming the conditions and still shooting well.
The night before, get plenty of sleep. Avoid alcohol and meals of food that you are not used to eating. Try not to eat too late; it might interfere with your getting a good night's sleep.
Arrive early enough to avoid the worry of getting there late and the stress that accompanies rushing your preparation once you arrive.
Make sure you know approximately when and if possible, exactly where you will be shooting.
Keep your supplies (shells, gun, etc.) as ready as possible. Also keep then secure to avoid worrying about their being stolen.
If you are shooting at an unfamiliar club, become familiar with the traps, their numbering system, where the numbers appear and how targets are thrown. You want as few surprises as possible.
Stay warm or out of the sun as much as possible. Try to find a comfortable place to wait before your squad is called. If you don't hear what is said on the PA system, immediately find out by asking someone who is sure to know.
When on the line waiting to shoot, use the same routine you have developed at other shoots and when practicing. Do things the same way. This is not the time to try anything new. (A written list will help while you are developing a routine.)
The "next" target is the most important but imagine that target breaking before you call for it. Imagery can be important. Avoid all negative thoughts and do everything possible to reduce or eliminate the fear of the next target or the fear of shooting poorly. If you don't it could become a self-fulfilling prophecy.
Use the eye and gun holds that targets and weather condition's dictate, changed as little as possible from the way you normally do things. Before calling for a target, imagine it breaking in your mind's eye and BELIEVE you will break it. (The power of positive thinking actually works.)
If a target is lost, do not spend much time analyzing why you missed it. If you know, plan what to do to avoid its happening again and continue shooting with confidence. Do not dwell on a lost target, even the last target on a field.
Once a shot is fired, relax. Stand as comfortably as possible without changing your stance very much, ideally, not at all. Think pleasant thoughts. Do not think about what you will do when you finish shooting, where you will eat or about the drive home. (Thoughts about scantily clad, blond, cheerleaders [or drum majors] are okay.)
Each target will be like one you have broken a hundred times before. Remember that and building confidence about breaking the "next" target will be easier - just avoid overconfidence and the risk of a sloppy or rushed shor.
Enjoy yourself. You are shooting. You love to shoot - you are doing something you love to do. Sure it's a registered or important shoot. So what? The world will not end if you miss a target or two and your dog will still love you.
There are no doubt things I have omitted and that others will provide.
Rollin