Mixing live sound for theater is a huge undertaking, and it's easy to get overwhelmed sitting behind a console with dozens of sliders. Running sound for shows requires a complex set of skills that can't be learned over night. Sound designers spend years perfecting their craft. However, for the novice, here are some tips for teching your Off-Off-Broadway show, showcase, or cabaret night.
1. Know the script. If you're running sound, you're going to need to know the lines of the show, not merely the cues for each scene or song. You should be able to speak along with the show. This will allow you to account for missed cues, but will also allow you to solve a problem (adjust a level, communicate with a stage hand), and still find your place in the script. Annotate the script. Fill it with notes.
2. Stay alert. Proper preparation is absolutely essential, but won't take away the need to constantly be watching everything. Unlike mixing your average concert, where you can take a "set it, leave it, and adjust" approach, mixing live theater will require new settings for each scene or song. And even with the most preparation, some things will be different for every performance. Microphones slide around. Monitors get kicked. Be ready to boost someone's levels, or to cut back a microphone that's feeding back.
3. Don't be afraid to use the tools at your disposal. If your system allows you to set presets for scenes, by all means, use them. Again, you'll still need to watch the levels like a hawk, adjusting constantly. Faders and filters can be preset, but actors cannot.
4. Communicate. Delegate tasks. Your stagehands should be reachable immediately, preferably through headset or walkie-talkie. They're here to assist you.
5. You can't control everything. It's impossible to eliminate all problems in a live scenario. Instead, work to minimize mistakes and errors, and more importantly, do not let them interrupt the flow of the performance.