Arrive at the venue early so that you have time for a thorough sound check. Before the musicians arrive, check that all microphones, cables and direct boxes are fully functional and that each sound source is routed to its own channel.
Place masking tape on the sound board and label each channel with the name of the instrument assigned to it. This way, if you have to adjust a specific sound source, you can quickly see which channel controls to use.
Place an on-stage monitor speaker close to each musician. Experiment with placement so that everyone can hear themselves clearly. If a floor monitor is too close, the sound will have trouble reaching their ears; if it is too far away, the sound will be lost among the overall stage noise. Taking the time to get an ear-pleasing mix for each musician will lessen the chance of them having to turn up their stage volume in order to be heard. As the on-stage volume rises, so does the chance of feedback.
Place stage amplifiers and speakers at the side of the stage. In the case of small combo amps, set them on a tilted stand so that their sound is directed toward the ears of the musician. This way, musicians can hear themselves play, but less of the sound will be directed from the stage and into the room. Ideally, the sound in the room should come from the house P.A., so that you can fine tune the levels of all instruments and voices. If the sound from the stage is too loud, it will negate the work you do at the mixing desk.
Set up a Plexiglas screen in front of the drum kit. These screens are extremely portable and come in sections that easily clip together. Since they're clear, they do not restrict visibility for the crowd or inhibit on-stage communication between band members, but they do greatly reduce the volume of the drums. As well as giving you greater control of the drum sound in the P.A., this also enables musicians to better hear their own instruments.
Work on the drum sound, tweaking the equalization (EQ) controls to tame any problematic frequencies that sound harsh or boomy. Overdoing the low frequencies can create a thunderous low-end rumble that will give listeners ear fatigue very quickly and will make it difficult to hear other instruments in similar frequencies, such as the bass guitar. Try running the drum mix through a compressor to even out the volume of the drum hits. This will make the drum kit sound smoother and give it a little sonic punch.
Add the bass to the P.A. mix and work on the EQ to get it to blend with the drums, while retaining clarity. This will take a little experimentation, but once you get the rhythm section to sound tight, the rest of the mix will be relatively simple. If the sound is a little muddy and you are having trouble hearing the bass, or a certain drum, try cutting EQ rather than adding it. Adding EQ also adds more noise and makes for a cluttered mix. Cutting a particular frequency in one instrument can often be enough to give it separation from another that is competing with it to be heard.
Bring in guitars and keyboards and blend them with the rhythm section until you get a nicely balanced mix. If you find that lowering the level of a guitar has no effect on the P.A. sound, it means that the guitar sound is coming mainly from the stage and you may have to ask the guitarist to turn down his on-stage volume. This is an occupational hazard of being a live sound engineer, and it can be awkward, but it is often necessary if you want to be able to control the sound levels in the P.A. and create a quality sound for the audience.
Raise the level of the vocal channels until they sit well in the mix. Vocals are usually the most important part of a live show, so make sure they are clearly audible. Again, use EQ controls to create space for them to sit. Cutting the lower mid frequencies of vocals will remove some of the boominess. Using a compressor on them is often a good idea because it tames the wild peaks and brings up quiet sections. This makes the vocal sound smooth and polished, just as it would in the control room of a recording studio.