It's easy to think the term "sound reinforcement" is another way to say "make it louder." But reinforcement and amplification are different things. The biggest mistake in live sound is just trying to make it louder. Clubs are prime examples of places where amplifying the sound usually just makes the music or band even harder to hear and understand. Reinforcing the sound by methods other than the volume control should be the goal. Proper speaker placement, becoming familiar with the frequency controls, mixing the stage monitors effectively with the house speakers, and communicating with the band are just a few of the simple ways to make the audio sound good and avoid the dreaded feedback squeal.
Speaker placement is essential. Every venue presents different challenges, so learning how to be flexible and adaptable is important. The main speakers should be out in front of the band's mics and monitors, and pointed away from the band. If the room is small, and the back walls are flat and bare, playing with the angle the speakers direct the sound will reduce the chance of feedback. When an audience fills the room, the "bounce back" is reduced significantly as well.
For the the stage monitors (the musicians' mix), place the speaker so that the back of the mic is pointed at the speaker. This takes advantage of the least sensitive part of the microphone so that the monitors are not feeding back into the vocalists mic.
Frequency controls are essential to learn for any sound engineer. Even the smallest sound mixers usually have at least a bass, mid, and treble control. Since every room is different, learning to adjust the frequency response to fit the room is important and a matter of practice. If feedback occurs in the high ranges, then reduce the treble. If the trouble is in the vocal range, experiment with the mid control. If the low end is making people uncomfortable, turn the bass down a bit. With frequency response, whether it is on the mixer or a separate equalizer, it takes practice.
After all the equipment is set up for audio and the band is ready, it's time for mixing. Getting the stage speaker (monitors) mix correct for the musicians is the first step. In Mix magazine, professional engineer Buck Moore points out that the engineer should leave the house speakers off for this part. Have each individual play or sing for basic levels, then begin to mix them all in one at a time, making sure they can still hear what they need. Depending on the gear, having more than one monitor "mix" can be very helpful. For instance, many bands put the vocalists on one mix and the instruments on another. Sending the different mixes to individual monitors can make a huge difference for the musicians and reduce overall volume, which is a good thing in most cases.
After the stage mix is done, the most difficult part of mixing is them performed. Individually add each instrument and voice in the house speaker mix and make individual adjustments.
The band and the engineer need to communicate. Being nice, patient and taking the time to explain things(and listen to the needs and concerns of the musicians) is an invaluable tool in the audio engineer's arsenal. When it's a team effort, everyone wins. Showing the band that making them sound good is the priority will make it a lot easier to do the job and be proud of the results.