20-some tips:
Do what the boss says without grumbles.
Always have the show memorized.
Being asked to turn up is better than being told to turn down.
Always arrive early and/or share the driving.
On stage, look at the audience, smile, keep an eye on the band leader and drummer, all at the same time!
If there are set lists, have a spare or two.
Stand on the right of the main guitarist (with right handed guitarist). You can then follow the guitarist's chords if you don't remember the changes. If you are on his/her left, you can't.
A rig with two speaker cabinets is more likely to shield you from driver failures, than a single cab rig.
Have a spare micro head that can cover the gig in your bag/truck/trailer.
Always have spare strings, straps, cables (including speaker/speakon), and batteries.
Have the tools you will need to adjust anything on your instrument.
Have a music stand and a stand light (either on stage or nearby) in case you are asked to read a chart.
Wear a clean good costume for the gig.
Wearing regular cowboy (riding) boots on gigs or on the road is going to hurt in the long run—Ropers are better for your feet.
Have a drum key in your bag and some spare AA, AAA, and fender medium picks—they are for the other guys.
A fresh but tested 12AX7 is a good idea, as is a flashlight, two pencils, and some music paper.
Be positive. Support all the other players, but stay away from the squabbles and gossip.
On the road, sleep as much as you can, drink less than the other drinkers, and don't hit on chick bartenders.
Have a good business card.
Learn the NNS and 5-string.
Get to know a lot of classic tracks and the new stuff too.
Be open-minded and drop your preconceptions about people.
Be deeply deeply grateful for all you have.