Personally, I avoid anything fizzy, 'cos burping's not very audience friendly.

Oh, yes, and know which key each song should be in, and sing in that key.

And don't sing too loud. In fact, if you sing a little bit more quietly, an audience will often be that much more attentive.

Know your material, which will help you to relax.

And most of the above re: water, no booze, etc.

Good luck!