Consider comfort and the balance with the music.
To make the arm and hand movements look beautiful, avoid puff sleeves. Also, avoid dresses that interfere with your performance. You should avoid high-heels since they could potentially make you lose your balance and concentration. Also, if you have audience, you need to avoid sneakers just because they feel safe. Some say the rubber on sneakers stop sound vibration (though it’s just a theory). But more than anything, they are too casual and do not look good.
You should wear something that you can easily put your whole weight on both heels, and the soles should be anti-slip and hard like leather to help a smooth flow of sounds. Also, don’t forget to carry a handkerchief.
Unless specified by the organizer of a competition, I don’t recommend wearing a tie that throttles your neck or shirts with hard collars. In the orchestra I belong to, wearing collarless shirts, polo-shirts, T-shirts or sneakers are prohibited when performing at concerts with a dress code of smart casual. When you need to dress formal, wearing a collar shirt that doesn’t interfere with your performance would be the best. Please choose something that is comfortable to play in, but not too casual.
This reminds me of the time when I organized a recital at a venue with a very strict dress code. Even the audience was prohibited from wearing shorts, jeans, sneakers, T-shirts, etc. I didn’t know about this and didn’t inform the audience beforehand, so many of them were refused entry. It was a very regretful experience for me.
This also reminds me of the story of a boy I know. He was only about 5 years old and he wore shorts to a competition. One of the judges wrote a comment saying “wearing shorts is not appropriate” and it surprised his parents. It’s good to keep in mind that things like this do happen so it’s important to always carefully read the outline of the contest. (Perhaps, it’s safer to always wear pants, just in case.)
Just like I said for females, don’t forget to carry a handkerchief.