Do you sing between gigs? Try singing around the house as you do chores, to keep your muscles in shape. You don't have to sing loudly.
Strong muscles have better control.

A choral director once told us that choirs go flat because people know that a low note is coming. They say 'Uh oh!' internally and then go TOO low.

Perhaps you are getting high because you know a high note is coming. You are worried about it, and when the time comes, you go too high. Give that idea a little thought, anyway.

And as Russ says, an occasional strum from an instrument should not be hard to do and will help you keep in tune.