Practice gives you the confidence in your ability to make the shot. Constantly practicing and watching your arrow hit where you are aiming builds confidence and trust in your ability to make the shot. The first time I shot 90 meters (98 yards, 2 feet) with my bow, my mind didn't believe I could hit the target. Now, after years of practicing at that distance, my mind not only knows I can hit the target, but it also expects my arrows to be in the middle of the target.
The best shot I ever made on a buck I owe to practice and the confidence that practice gave me in my ability to make the shot. Using the bow setup I had at the time, I could hit a 1x2-inch sticker out to 60 yards. One day, after a couple of hours of shooting at and hitting this sticker, I was out hunting and a nice 130-class 8-pointer came by. At his closest point, he stopped at 40 yards as confirmed by my Nikon rangefinder. He was completely covered by brush except for a 4-inch open gap behind his shoulder.
As he stood there, I realized that sending my arrow through this 4-inch gap was much easier than hitting the 1x2-inch sticker that I'd been shooting at several hours earlier. Additionally, I realized that if I for some reason missed, I would hit the brushy cover and not injure the deer. After taking a deep breath and concentrating, I sent the arrow through the gap and made a perfect hit on the deer. Had it not been for my practice, I never would have had the confidence to attempt and make the shot.
Make your practice sessions as close to actual hunting situations as you can. I always practice with my hunting bow while wearing the same warm, bulky clothes I wear in the field. If you anticipate shooting from the sitting position, practice from the sitting position. The closer your practice resembles actual hunting conditions, the more comfortable you'll feel making your shot in the field.
Try to simulate shooting under nervous conditions. Most hunters are completely calm during practice, yet they often get very excited when presented with a shooting opportunity in the field. Many hunters experience increased heart rate and breathing. Simulate these conditions in practice. Run around for a few minutes until you feel your heart rate and breathing increase. Then go pick up your bow and try shooting.
Make it more interesting. Mess with your mind. Pretend you have just been presented with the buck of your dreams and this is the only shot you'll ever have at him. Now take your practice shot. If you can learn to successfully hit your mark under these conditions, you'll have the confidence to get the job done from your tree stand when it really counts.