This is from an athletic performance expert, but the same mental focus issues and nerves can affect a musical performance. We've often complained how we can have a certain type of show and we'll psych ourselves out before we even take the stage, basically failing before we've even started. This addresses that:

I receive several questions each week from athletes who want to learn how to cope better with pre-competition jitters or pre-event anxiety. I thought this would be a good topic for this month's newsletter as pregame jitters apply to all sports and all levels of performance.
What are pre-competiton jitters? Jitters are feelings of excitement or butterflies in your stomach prior to the start of competition. But not all athletes experience jitters in the same way. Why? Two types of pre-competition jitters exist - positive and negative jitters. The first is the friendly kind of jitters characterized by excitement and anticipation, all good stuff. This is typically a feeling of positive anticipation at the start of competition.
With the good jitters, you feel excited and ready to get the game going; you want to play right away. This positive type of jitters can help you perform better by helping you heighten your focus. You feel ready, adrenalin is flowing, and your focus becomes more acute. The pros often experience pregame jitters and interpret them as a necessary part of sports.
The second kind of jitters, negative jitters, is the type that makes you have a sinking feeling in the pit of your gut. You have butterflies, but feel like throwing up. Your mind races, heart rate accelerates, palms sweat, muscles tighten, blood pressure increases, and you get an uncomfortable feeling in your stomach.
Negative thoughts or worry races through your mind as you worry about embarrassing yourself, not performing up to your ability, or feel intimidated by the competition.
When you feel anxious jitters, your performance suffers because it makes you physically tense and cripples the ability to focus. When an athlete experiences anxious jitters, he or she become even more worried that something is wrong mentally and physically, which then turns into a bigger distraction and more anxiety.
You actually want to embrace the first kind of jitters because they will help you focus and give you the energy to perform your best. Pros want to feel the friendly jitters because they know this is a positive sign that they are ready for competition. If you get the second kind of jitters, this can be hazardous to your performance and you need to learn how to cope better if this sounds like you.

What is the solution to the negative or harmful jitters? The first step is to discover what the real fear that is holding you back. What are you anxious about? Here are some likely options:
Fear of embarrassment - don't want to look stupid in front of others.
Fear of performing poorly - don't want to get off to a bad start.
Fear of losing the match - afraid to lose the game to a rival.
Fear of not playing up to your own expectations - afraid of not achieving your capabilities.
Fear of not being respected - worry about how others will perceive you as a person and athlete.
As you can tell from the list above, many fears come from worry about your level of performance and what other people may think about your game. Sometimes these fears are irrational and not based on reality. Sometimes they are based on past experience and how you felt after a lose.
The first step is to do a reality check with your own fears. What's the worst thing that can happen if you get off to a poor start to the game or match? No one is going to judge you on how you perform on the athletic field, but it feels like that for many athletes.
Try putting your fears aside and focus on something more pleasant. Keep your mind distracted from unpleasant feelings you have because if you give them energy, you will most likely feel more anxious. Relax and talk with you teammates or playing buddies before competition. Try to stay light and not so serious about the day.
Second, know that pre-competition nerves are part of sports and that all athletes get them. You must interpret them as helpful and useful to getting you focused for the game with a shot of much needed adrenaline. This is how the pros look at jitters.
Focusing too much on the uncomfortable feelings only heightens the tension. Don't bring more attention to the knot in your stomach or a rapid heart beat when you feel the unfriendly jitters. Shift your attention to your game plan and how you want to play the first set, hole, or opening minutes of the game.
Try to relax and talk with your teammates or playing partners. Some players work themselves into a state of anxiety as they stand around and worry about results before the game even starts! Keep your mind occupied with relevant thoughts in the present moment.
Lastly, keep your mind focused on execution, instead of results. You cannot win the game in warm ups or in the opening minutes of the game. Ask yourself, "What do I need to do to perform my best today?" Think about what you have to do to be successful instead or worrying about mistakes or embarrassment. Worrying about mistakes will only increase your anxiety.
When it is time to play, don't rush, slow down. Above all else, take your time during your warm up or pregame routine and think confidently! You don't want to just go through the motions physically. This is a good time to give yourself a pep talk with statements of confidence and composure, such as "I know how to play this game, trust your practice to get the job done today!"

Now we know that one of our biggest issues is that we're not practicing regularly. That's one weakness that we're going to have to shore up before we can confidently move forward. Bottom line.