Here's a few tips for budding sound engineers working that I've learned from being on both sides of the desk over the last 13 years. It's directed at people working small clubs, but plenty of the concepts still hold for other contexts.
I'd be keen to hear your thoughts. I think I'll write a similar type list for performers too.
1). Turn up on time. When a band thinks sound check is at 2:30pm, turn up at least 30 minutes (preferably an hour) before. Use this time to make sure the system is running properly, EQ monitors, FOH etc. Get out your stands, mics, leads etc, and take stock of what gear you've got to play with.
2). Communicate with the band. Get an idea of what kind of sound they're going for. If in doubt, leave the large multi-tap delays off ALL instruments unless specifically requested.
3). Try and be as fast as you can. If everything is set up beforehand, soundchecking shouldn't take too long. Bands HATE sitting around waiting.
4). Make sure the band is happy with their stage sound. Write down the settings for their monitors so you can recall them later. I don't really see the point in getting a band's monitor mix organised in soundcheck, then forgetting to apply it when they play. Digital desks have made this much easier.
1). The key word here is "sound reinforcement". When setting up a mix, listen to what's coming off stage before throwing up faders. Try and get everything balanced by using as little through the PA as possible. Pretty much your number one priority is getting the vocals and DI'd instruments clearly out the front. Support the mix with low end from the drum kit. This should be your base. Front here you can start increasing the overall level of the mix, adding guitars to taste etc. A classic mistake I often hear is bleedingly loud drums & bass, some vocals, and no guitars. It sucks to listen to.
2). Keep your eyes on stage, watch the performers. It's highly likely they're going to be communicating with you with requests for changes in their monitors.
3). Unless you know the band's music inside out, don't start changing levels when it appears band members aren't singing/playing certain instruments. If you find yourself having to back faders off when people aren't singing, you haven't done enough preparation getting the system EQ'd properly.
4). As a rule in small venues, don't gate anything. You don't know how dynamic the drummer is going to play. If you're having trouble with the kick drum feeding back, go back to your system EQ and fix the problem. Sometimes the drum might need re-tuning, but it's unlikely. I find myself telling young engineers to turn the gates off all the time. Gates and expanders are for big kids who know what they're doing.
5). Try not to have it screamingly loud. If you notice several people in the audience with their fingers in their ears, it's probably too loud.
6). If musicians are complaining about feedback in their monitors, it's your #1 priority to fix it. Use your headphones, work out where it's coming from, fix the problem. Same with dodgy leads. Sometimes this means getting out from behind the desk, and solving the problem onstage.