Hi Jed,

Drink (non-alcoholic) fluids until your kidneys are floating. You may have to pee a lot but it will help your voice. If congestion is interfering with getting fluids down, drink them warm, which is better for your voice anyway. Then if you need it, have a single shot of whiskey or brandy right before your performance and chase it with lots of room temperature water. I've been there: you can sing if you are hydrated enough.

Best of luck.

Sharyn