Having a winner's posture will give you confidence. Wearing nice clothes, sitting upright, walking confidently, breathing calmly, etc will have a great effect on your mind as well as your look. It will also make you sound better because you'll stand or sit better and have a winner's attitude.
There is a lot we can learn from people who are frequently on the spotlight. It is very useful to look at how they stand, walk, talk and try to replicate that in your own way. You can imagine that you are a celebrity walking in a talk show and adopt his/her posture and attitude. "Act" like this for a few minutes and then pick up your instrument and play like the star that you are!