Routine, routine, routine. This is a great way to minimize nervousness. To make it effective you must do EXACTLY the same things EVERY time. At some point in your preparation, as you start to set up to your shot, follow the same steps, using the same amount of time, every time. Same number of practice swings, steps up to the ball, looks at the target, waggles, etc. It's like autopilot for the golf swing. Once you have decided on the shot, stop thinking and react. If you have doubts, start over.