I was going to mention bananas (for potassium) too.

Warming up before you play helps...treat yourself like an athlete.

you have to warm up especially if you're doin lots of stretches and barre chords. I don't have a band but play regularly in a local acoustic open mic night, one of my fav songs to play is Message In A Bottle. It's a nightmare on an acoustic with the stretches and barres and the only way I can do it is by playing somethin easier before it to warm up.

The FIX TO CRAMPING HANDS...
 

I have been playing for over 20 years and I have had this hand/forearm cramping happen more and more over the last few years.. I have pin pointed it to times when I have either had an active week (gym, sports etc..) and not drinking enough water... especially combined with drinking booze..
Now I won't pretend that I don't like having a few pints when I play.. However.. if you hydrate properly.. you will not have this issue.. this is 99% an issue with you simply being dehydrated and your muscles letting you know... you left hand is working overtime and then it cramps.. try drinking beer all day and playing some sporting event - (beach vball, ball hockey etc...) and not hydrating properly...your legs will do the same things...) (it will be fun though!)
Drink LOTS of water every day... or at very least.. 2 - 3 days before show.. and cut back on your beers during the show) I usually ramp up on gatoraid a day or two before.. ad even bring a few to the show to sip on instead of a beer. NO MORE ISSUES...
have fun.. and happy cramp free playing!
BQ

I just wanted to throw this out there...

If you are on any medications, particularly psychiatric drugs, this could be the root cause or a primary factor.

I have, what is for me, perfect form (in other words, it is not perfect but is comfortable to me and doesn't strain myself). Then one day my fretting hand started to cramp up on me whenever I would play. As time went on, it took less and less time for it to cramp.

It turned out to be caused by the medication I was on.


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Once I realized this, every gig got easier and sounded infinitely better.