My first shot at theatre sound was my senior year of high school. The show didn't have a sound tech, the guys in stage crew apparently never saw a console before. The music director was one of my teachers, he knew i was doing sound at my church, so he conned me into doing the musical. It was kind of funny. I show up 4 days before opening night, and try to learn the show, and how to mix so many lavs at once. The guys had the console patched backwards....channel 1 on the snake was going to 32 on the console, and so on. I spend the first day repatching and verifying what actor had what mic, and what PZM was going where.
Lesson #1: Plan an input list. Go through each page of the script, identify the leads and main characters, and figure out who needs reinforcement throughout the show. If you are in a situation where the lavs are scarce, and the actors are many, pre-plan when you can swap mics between actors. Which leads to...
Lesson #2: NEVER NEVER NEVER trust actors to handle, put on, turn on, change batteries, turn off, mute etc. wireless microphones. My first show was a disaster because I trusted the actors (my friends) to make sure people got the right mics, and make sure they were turned on. Chances are you can find a member of the stage crew, or a stage manager to handle any mic swaps, or changes to the mics during the show. Hopefully you have some sort of intercom system so you can talk with this person backstage while you are at FOH. Make sure they know without a doubt how the mics need to be on, not muted, and even how to change frequencies on the mics if some problem arises mid show. After I learned this lesson, I had a runner just for audio to handle issues during the show. Over the com, I was able to talk about mic placement issues, battery issues, etc. and keep the show going without a hitch.
Lesson #3: Know your cues! I got in the habit of taking my copy of the script, and noting all changes during the show. At the bottom of each page, were warnings of up coming cues, for muting mics, sound EFX etc. I didn't have a console with recall back then, so this was all manual changes. What I ended up doing was having another person sit next to me during the show, calling the audio cues for me. This person read along each line of the show, and when a cue was noted, he called it out to me, so I could keep my hands on the console, concentrating on mixing, and he worried about the upcoming cues. The cues would be something like "unmute channels 3,7,9....mute channels 5,6,8. First line is on channel 2." That kept me prepared for each new scene, or character coming in mid scene....
Lesson #4: MUTE ALL MICS THAT ARE OFF STAGE!!! The show was having a bit of a rough go. One of the leads was really struggling. Lines were missed, lines were early, the scene was a mess. The first thing the lead did when they left the stage was head right to a room off stage, and proceed use every expletive in their vocabulary....They got 4-6 words out before I figured out who it was and could get their channel down....not a good scene!
All in all, after all of the preparation is put in, the shows should go relatively easy. Being prepared will give you the mental capacity to deal with problems when they arrive.
Now that digi consoles are becoming more common, it makes it easier to program the show and just hit the "next" button. By all means use these functions, but remember how to take over and do it live if you have to. Have fun, and try not to get the show tunes stuck in your head.