I'll keep it basic to start:
- Your outfit should be neutral - I should remember you and your singing, not your clothes.
- Your hair should be out of your face, so I can see how you use your face expressively.
- Coat and tie are not necessary, but you should dress in such a way as to be respectful of the situation - and no jeans!
- If you're a mezzo or soprano, you don't have to dress androgenously to sell Cherubino, Octavian, Oscar, Seibel, or any of their brethren.
- Don't underestimate the comfort factor - being comfortable - clothes not too tight, shoes that allow you to balance, etc. - all are important in letting you do your best work.