Glenn
kbrkr got it right.
Warm up before the gig(s). If a have the time, I'll run through a variety, maybe 4 or 5 songs that take me through the necessary ranges, a few hours before the gig.
Also, on the way to the gig, in the car, gently sing along with the radio or whatever.
Run through some vocal scales, and when you reach the top..slide down to the lower octave. Likewise, doing the scales in reverse, when you reach the bottom, slide up to the octave.
You need to loosen up the vocal cords.
Don't drink anything too cold, and as unpopular as this notion is..No Alcohol. A good warm up is essential Glenn.
I'm 52, still gigging on a regular basis in a solo act, plus I'm playing with a Show Band, covering everything from the 50's through the 80's. So far I haven't had any problems.
Good Luck..Hope thisd helps
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...L