As much time as you spend in your rehearsal space perfecting your sound, it won't mean anything if it's botched coming out of the PA. All the money you spent on new pedals, amps, guitars and strings doesn't matter if the mix is off in the club.
The sound guy (or gal) is the most important component of your show that most bands don't really think about. He (going with he for this piece out of ease - and most are men) can break your set (few sound guys can actually MAKE your set if you suck).
So, you have to know how to approach sound guys right and get them on your team for the short amount of time that you have with them.
Get His Name
The first thing you should do is introduce yourself to the sound guy when you arrive. Shake his hand, look him in the eye and exchange names. Remember his name - you're most likely going to need to use it many many times that night and possibly a couple times through the mic during your set. If you begin treating him with respect from the get go he will most likely return this sentiment.
Respect His Ears
All sound guys take pride in their mixing. Regardless of the style of music they like listening to in their car, they believe they can mix any genre on the spot. However, most sound guys will appreciate hearing what you, the musician, likes for a general house mix of your band's sound. Don't be afraid to tell him a vibe or general notes ("this should feel like a warm back massage" or "we like the vocals and acoustic very high in the mix" or "we like keeping all vocal mics at about the same level for blended harmonies" or "add lots of reverb on the lead vocals, but keep the fiddle dry"). He'll appreciate knowing what you like and will cater to that. He is most likely a musician himself, so treat him as one - with respect. He knows music terms - don't be afraid to use them.
Don't Start Playing Until He's Ready
Set up all of your gear but don't start wailing on the guitar or the drums until all the mics are in place and he's back by the board. Pounding away on the kit while he's trying to set his mics will surely piss him off and ruin his ears. Get there early enough for sound check so you have plenty of time to feel the room out (and tune your drums).
How To Insult Your Sound Guy
Call him "yo sound man" if you want to piss him off. You got his name, use it. Or ask him politely again if you forgot. Don't tell him that the house mix is "off" or "bad." Everything is subjective. It may not be what you like, but it's obviously what he likes. He most likely has WAY more experience mixing than you do. So get specific about what you like and don't like for your band's house mix from the get go or shut the hell up.
Know Your stuff
Know how you like your vocals EQed generally so you can say that. You can say "can we drop some of the highs on the vocals in the house." You shouldn't say "the vocals sound piercing - they hurt my ears." You should know how your gear works inside and out so if anything goes wrong you point to the sound guy last. Pointing to him first is a sure way to piss him off.
He's Part of the Club
The sound guy, door guy, bartender, booker, managers and servers are co-workers. Just like you and your fellow baristas are co-workers. They hangout, have work parties, hit the bars together and they talk. If you're a jerk to the bartender he'll tell the sound guy and the sound guy may then decide to mess up your set out of spite. Or just not put any effort into mixing you.
Everyone Wants A Great Show
Believe it or not, your sound guy wants to perform at his best just like you do. Make his job easy by showing up prepared and not sucking. He most likely has his stuff together so make sure you have your stuff together as well and don't mess up on stage.
There are sound guys out there (we've all worked with them) who seem like they have a massive chip on their shoulder from the moment they step in the club. These guys are typically older, failed musicians who have been at this club for decades. They are hardened from years of working with jerk musicians who not only suck, but believe they are rock stars and that the sound guy is a peon - and treat him as such. You may not be able to change his outlook on life, but treat him with respect and dignity from the get go and he may lighten up just enough to put some effort into mixing your set.
Even though it should go without saying, use the golden rule. If you treat your sound guy as you'd like to be treated and work WITH (not against) him on putting together a great show - you most likely will have one.