| This is probably too late, but maybe it'll help someone else...
Warming up your voice is the single best thing in my opinion. I personally try to get warmed up the day before, and then immediately before getting on stage (which isn't always possible). If you're not sure if you can warm up your voice before yo9u go on stage, do it sometime earlier that day. Scales are a great way to go - the same way you do with your bass, except do vowels. Go A- E -Ah - Oh - Ooh - that covers the basics of vowels.
There's also a ton of Youtube videos with warm up exercises. Watch a few and follow along.
Also, avoid dairy (even cheese), cold drinks (yes, even beer), as they both mess with your trachea and your vocal chords. As mentioned before, lay off the smoking (of anything), if you can. Dairy coats your throat with fat and enzymes that doesn't come off too easily, and sticks to your vocal chords and throat. It's sort of like if someone poured a soda all over your bass making it all sticky - it's tougher to do your job. Smoking and cold drinks constrict the muscles and other tissues in your throat. It's analogous to having hand cramps and then trying to play bass properly. |