The title is a bit tongue in cheek but the purpose of this post is to help you get rid of some of the aggravation that comes with having to talk to people while mixing. Of course those of us who mix in bars will still get that one drunk that thinks you're in the band and wants them to play a particular song. Of course stage mothers will still come up and holler that they can't hear their little Billy. But for people you work with all the time there is etiquette to talking to a sound guy and if you're not up to educating people just send them this post.
The correct way to address a sound guy while he's mixing is to approach from the side and slightly behind. If he's clearly actively mixing wait a few seconds to see if you can get his attention. If it's something vital like a breakdown or a musician signaling for a monitor mix fix then by all means a tap on the shoulder is acceptable. Remember to stay just slightly behind though. (There's nothing like being responsible for a large console and having half of it unreachable because someone is standing with their belt buckle on fader twenty-four and you're stuck at the master section.)
Keep in mind that even if a sound guy doesn't look like he's tied up, he's likely responsible to keep track of at least four musicians and a dozen channels of audio at the bare minimum. He might not be frantically turning knobs but if he's worth his salt he's still actively engaged with what's going on up on stage. Say what you have to say with as few words as possible, wait for a reply if needed and back off. It's that simple.
I'm pretty fortunate that people where I work just seem to get this. Everyone from the head pastor to the junior intern approaches me this way and I never asked them to. They're just polite, considerate human beings. Some have even apologized for interrupting but I always make it clear that input is welcome, especially if it's about something broken that I'm not aware of.
It's a two way street Brethren of the Knob and Fader. There are many in our field who confirm the image of the testy, uncaring sound guy. It's up to us to put everyone we work with, from the musicians to the crowd (and yes, even that annoying stage mother) at ease. If you get complaints, don't make excuses, say thanks for the information and then do your best to fix it. Run your venue like a gentleman and you won't have to defend yourself. People who already know and love you will do it for you.