An advance is a summary of details the club or promoter should send to the bands at least a week before the show. It tells everyone when to load in, when they will sound check and how long they will play. A detailed advance prevents confusion in the hours before the show and keeps everyone focused on the task at hand.
Sound engineers need to know who they'll be dealing with and what their needs are. Before the show, make sure you know the names of the bands, which instruments they use and whether they have special needs (a horn section, a choir, pre-programmed beats, in-ear monitors).
When you meet the bands, introduce yourself and impress upon them the show will run well, run on time and that you're working to make sure everyone sounds great. Ask the bands to keep their stage volume low and explain their equipment will sound better and more balanced coming through your system than it will on its own, at blasting-loud volume levels.
Bands are responsible for loading equipment on and off the stage efficiently, but they'll work even faster if you're providing an extra hand. If you have time between adjusting microphones and setting up the stage for the next act.
Backlining (leaving gear onstage for the whole show) is an ideal time-saver and an easy way to make sure you're getting consistent volume and sound throughout the night. There are two ways to backline: the built-in backline, where every band uses the same gear (often provided by the club or the promoter), and the rolling backline, in which the headliner sets up everything first. The rest of the bands place their gear onstage in reverse order, with the opening band's gear out front. The exception is with drums; often it is impossible to backline more than one drum set, or two if the stage is big.
A great show runs on time. Make sure the opening band hits the stage on time (or, on the fly, at the time when the promoter wants to start the show). When a band has about 10 minutes left, tell them so over the monitors. This will help prevent them running over on their set time. One late band makes for two late bands, which makes for the headliner going on late and having to play a shorter set. This is a disaster, especially with headlining bands that have contracts requiring a specific set time or length.
Live sound comes down to four components: The band's gear, the band's performance, the club's PA and your placement of microphones. In general, the more mics, the better, but at minimum, mic the kick drum and all the vocalists. If your room and PA are so equipped, mic guitars, run the bass through a direct input, mic the toms and cymbals (and the snare, if the drummer is a light hitter). You will learn through experimentation which microphone scheme works best in your room, as every venue is different.
Every sound engineer needs a flashlight on his belt, a full complement of tools and plenty of extra cables--quarter-inches, speak-ons, mic cables, everything.