Solo performances can be very nerve-racking. Even if there is only a judge there, it can still get very intense. Solos in front of large audiences are also daunting. Preparation is extremely important for a solo performance. Nothing can compare to the feeling of messing up a piece while you are on stage!
Know your music in and out. Be able to play it with your eyes closed (if it is shorter). Be able to play the dynamics without paying attention. Make your piece of music a habit. When you know it very well, you will be able to play it musically even in front of an audience or judge.
Take deep breaths. Deep breathing is obviously very important for playing a wind instrument, but it also helps to calm your nerves. Press your diaphragm out, filling up your lungs to full capacity, and release slowly.
Prepare what you are going to bring beforehand. This might be a given, but often when people are nervous, they tend to forget things. Get all of your music and instrument and put them in a place where you cannot forget them.
Keep warm air going through your instrument. This does not apply to pianists. Blow into your instrument so that you stay in tune. When you warm up, you are actually warming up your instrument. This temperature is important for clear intonation.
Keep good posture. Your posture can affect sound quality as well as intonation. Also, since you are in the spotlight, you want to look the part. Stand or sit up straight. If possible, you can move with the music to add emotion like swaying or moving you instrument slightly. Do what feels comfortable.
Enjoy playing! Always, remember that you are playing because you enjoy it. Get comfortable with being up on stage. Have fun with it! And if you mess up, it doesn't matter because everyone makes mistakes and that will just tell you to prepare harder next time.
Solos can be very enjoyable! However, that can only happen when you prepare and make it enjoyable. If you mess up, just make sure to learn from your mistake and don't make the same mistake again! I wish all soloists the best of luck in all endeavors!