When most people think about concerts and playing out live, the things that lead to mind are the musicians — the singer, the drummer, the guitarist. This is not a bad thing; the band is the primary reason why the concert is happening in the first place. But, it's not only the band that puts a show together.
No band is successful if the team consists only of the musicians. Every band needs support staff: roadies, agents, managers, accountants, lawyers, vendors, etc. And, while all are important, necessary and deserving of respect, I'm going to take some time to single out a few of them (the ones that most people will encounter during their musical career) for special attention.
This is probably the single most important member of the support staff; he (and for some reason, almost all of the sound people I have encountered are male) is arguably more important than some of the members of the band. He has final control over how the band sounds to the audience. No matter how hot the band is — you could have Miles Davis, Jaco Pastorius, Jimi Hendrix, Aretha Franklin and Neil Peart on stage, all on fire, in sync together, everything's perfect, but the band will sound like garbage if the soundman's zoning out, not paying attention to things with the 2k Hz rolled all the way down and the 500 Hz all the way up on the master parametric EQ. A good soundman can make a bad band sound good, while no band will sound good with a incompetent moron running the desk.
If you happen to find a great sound guy with a good ear, hang on to him for all he's worth. Buy his gear if you have to, but soundmen with golden ears are irreplaceable. Which brings me to the next point....
While it's generally a good rule of thumb to be as nice to people as you can, it applies even more so the sound guy. No matter how much of jerk he might happen to be, no matter what has happened to you, never take it out on the sound man. Or, at the very minimum, wait until the performance is over and you don't have to deal with the guy anymore.