I hope this is not some sort of trap. If you want some advice, don't worry. (Yeah, it sounds cliché.) As long as you practice enough on whatever piece of music that you have to perform (or just your instrument itself), then instincts will take over. The more you worry/fear that you are going to make a mistake, the more likely you are going to let it mess with your head when playing in front of the judge. I have learnt that from a ton of experience.
So... don't let the stress or fear get to you. Finally, don't pay attention to the judge when performing. My listeners almost always make me loose my concentration when I look at them.
I hope this helps. If not, then I hope you laugh at my failed attempt (because laugher sooths people).
