Here are some of the things I try to do before rehearsal, soundcheck, and “show time” every time I run sound:
Show up on time. Actually, I recommend showing up early- make sure you have plenty of time to do your job and account for any curve-balls thrown your way, if this means you need to come earlier than was requested of you than you should plan to come earlier.
Look over the run sheet, make sure to know what’s going on and not be surprised by anything.
Plug in all needed mics and equipment - whether this is one mic or an entire system, make sure it is done and in a timely manner.
Run line checks: this includes testing all of the mics, DI’s, and line inputs (videos, iPod, etc). This can be done with or without the musicians; choose the right way to do it for your situation, just make sure that everything is working prior to the start time.
Clean up the stage, make the cables nice, tape down any areas that will be high-traffic locations or possible trip hazards.
Ring out any mics that may be on the edge of feedback - this is most commonly needed with choir mics and speaking mics (pastor, podium, etc). However, don’t ring out mics or speakers that are not close to feedback - you are more likely to make the sound worse, not better.
Find your feedback thresholds for any mics that you might have to run up right to the edge of their limits, especially ones that are important (i.e. the pastor, lead vocal, etc). I like to set the gain on these mics so that ringing and feedback is just starting between +5 and +10dB on the fader, that way you can safely leave the faders at unity and then turn them up that extra little bit when needed. This also gives you the finest resolution on the fader and thus the easiest way to keep the mics at their loudest possible volume without going into feedback. Even if you don’t do this, make sure you know (fader location) where the ringing/feedback will start, and then don’t go past this point.
Preset the console. Musicians will thank you if they can start rehearsing without a long soundcheck:
Label your channels.
Patch/route everything.
Get your gain knobs in the right range (obviously a dynamic vocal mic is going to need more gain than a drum mic or a keyboard DI).
Preset some eq and compression settings (based on past experience).
Preset your effects sends levels and effects settings (leave the returns off until actually mixing).
Know what channels you want to use any gates, de-essers, etc on so that you can quickly work on them once you are running soundcheck.
Preset your monitor mixes. Once you have worked with musicians enough you can usually get the hang of what they want and save a lot of time by presetting things.
Check in and communicate with the “important” people - worship leader, pastor, tech director, etc; really whoever might be in a position over you and/or working closely with you.
Let me know in the comments what else is important for being prepared (flashlights and multi-tools anyone?).