I was the FOH engineer for about three years at an all ages club in Southern California in the late 90's. We got everything from local high school bands to touring international bands.
Here are some of the things I learned.
Wear earplugs. You can get musician's earplugs custom fit to your ears with a range of Etymotic diaphragms to attenuate to different levels. They sound much better than the cheapo foam earplugs, and the frequency response is much better.
Create a CD full of songs that you are familiar with and that represent different sound elements (e.g. cymbals that are on the verge of too bright, standup bass that sounds great, etc.). If you're not using pink noise, you'll want to set up the house EQ on the sound system so that it sounds good to you. You also need to ring out the system by having someone on a stage mic talk into it, and then raise the volume slowly until it feeds back. Now notch that frequency down on the house EQ. Repeat this a few times. You have just increased the effective bandwidth of your sound system tremendously. There are also fancy devices that do this for you.
A live mix is really only half of a mix. If you turned the sound system off entirely, the room would not fall silent. Therefore, your job is to fill in the other half of the sound so that the room gets a full, correct mix of the sound system and stage volume from the band. This is why if you ever hear a recording taken straight off the board in a club the vocals sound way too loud.
Stage volume is the enemy. There is an inherent minimum volume that a rock band can play at. That minimum volume is dictated by the drums. Since they are a loud acoustic instrument, they will form the lower bound of how quiet the band can be. I always told the bands to try and match the drummer in terms of volume.
You want to avoid volume wars, e.g. everything louder than everything else. Bands never ask you to lower the volume of anything, they only want to raise the volume of an instrument, usually the one they are playing. Sometimes if a vocalist asked for more of themselves in the monitors, I would simply notch out the high mids on the guitars in their monitors, giving them the perception of more volume, which is what they really wanted after all.
Many mixing engineers are somewhat baffled by monitor mixes. These are very important. During soundcheck, after things were pretty well dialed in, I would always walk up on the stage while the band was playing through a song and stand by each band member in turn, making note of how it sounded. A lot of bands, especially less experienced ones, can't yet articulate exactly what they need in the monitors, but if you can help them hear what they need to hear (generally the band, with an emphasis on themselves) they will play better and love you.
Some bands are just going to be jerks, no matter what you do. Read "How to Win Friends and Influence People". It helps. Live sound is paid, on the job, assertiveness training, whether you like it or not. You can't win 'em all, but if you do well you can win most of the time.
When you do a soundcheck, write down all of the settings. If there are any opening bands or if anyone monkeys with the board, you need to be able to recall your settings in a repeatable fashion.
During soundcheck, you will have more reverb and high end sound in the room, since the crowd isn't there yet. Plan for this and mix a little on the bright side during soundcheck, since people are basically sound absorbers.
Even if you're mixing industrial, punk rock, or death metal, never let the mix sound harsh. Mics like the Shure SM57 and SM58 have a presence peak around 4kHz. If the majority of the mics on the stage are like this, the whole mix is just trying to cut through itself, and it's way too much.
The first song when the band goes up on stage is probably going to be the roughest. Maybe the guitarist changed their settings since the soundcheck (but hopefully not). To a certain extent, anything can happen, so you have to be ready to dial things in quickly. During the first song, the first priority is to make sure that you can roughly hear everything. The next step is to solo each track and make sure the gain settings on the VU meters are consistent with where you set them during soundcheck. You have to play "search and destroy" with noise, and the best way to do this is to go channel by channel and try to EQ out frequencies that are detracting from the mix. For example, if a guitarist has a lower end amp that is WAY too bright and harsh, don't be afraid to apply drastic corrections at the EQ. Remember, you're only doing half a mix, so overdo it a bit in the other direction so it balances with the stage volume. The larger the room, the less drastic you should be with these things.
Sometimes you just can't win. I remember many punk rock shows I mixed where one band would sound awful no matter what I did. Then the next band would get up on stage with an almost identical configuration of players and instruments, and sound great with almost no effort. Frankly, if the band sucks, you can only do damage control. If both guitars and the bass are all competing with each other in the same octave, their amps are all cranked up way too high, the drummer is light on the kick and snare but heavy on the cymbals, and the singer alternates between whispering and screaming, there's only so much you can do. Compressors on the vocals can at least force things into the right volume range if you're doing total damage control.
On average, metal bands are more egotistical and hard to deal with than punk and ska bands. Goth and emo bands were usually highly reasonable. There are always exceptions though.
You have to "own" the stage, and make the bands respect you. At least with the type of music I was mixing, the bands would sometimes damage the equipment. They have to know that this is not OK, and they will have to pay for it. I don't suppose this applies to anyone mixing church services.
If you don't know already, learn how to wrap cables properly. Overhand, underhand.
Don't let your amps go into thermal protect mode. If you drive them too hard, you will blow speaker drivers, or the amps will shut down for a few minutes, or both. If it's summer and the place is packed and the amps are on the edge, sticking a fan near the amp rack can get more mileage out of them. I never blew a speaker driver, but many of the engineers that came before me blew out several. They had a whole row of blown out 2" compression tweeter drivers up in the sound booth, with Sharpie writing detailing how they died and who was responsible.
Live sound was both great and terrible. When it was fun, it was really fun. When it was work, it was really work.