A few years ago a good friend gave me a book by Michael Talbot entitled The Holographic Universe. Here is an edited quote:
"In one study a phalanx of world-class Soviet athletes was divided into four groups. The 1st group spent 100% of their training time in training. The 2nd spent 75% of their time training and 25% of their time visualizing the exact movements and accomplishments they wanted to achieve in their sport. The 3rd spent 50% of their time training and 50% visualizing and the 4th spent 25% training and 75% visualizing.
At the 1980 Winter Games in Lake Placid, the 4th group showed the greatest improvement in performance, followed by groups 3,2 and 1, in that order."
I'm not suggesting we spend 75% of our "trumpet time" visualizing on a daily basis, but doing so on the day before the big concert might be a good idea.