Alot of really good info here, I've a few things to add.
If you get a good drum sound,everything else will slide on top. I find a big drum sound with loud clear bass makes even the shittest of guitar tones sound nice. The sickest guitar tone in the world will sound shit with poor sounding drums.
Talk to the band about their on stage sound/volume. Alot of times they're clueless as to what level they should be at and just go for it. If you leave them play for a minute then stop them and ask what they can and can't hear, work from there. Comfortable bands- just like in studio, perform alot better. In turn the sound is better and everyones happy. Clever stage set up in smaller venues can go along way to keeping levels down and monitors at a minimum, Like how is the drummer supposed to hear whats going on if the ampsare pointing away from him and are 5 feet in front of his kit?
Ride the guitar faders between vocal and instrumental parts, bringing the guitars down a hair during vocal parts can really make the vocals pop out and if you ride the faders back up then no one will even notice that the guitars were a fraction quiet.
Boost the mids on a guitar solo instead of just cranking the volume a little of both I find is just the ticket for making a solo stand out.
If you watch the musicians you'll quickly get a feel for when they'll do stuff, after a while you can catch almost every solo or scream before it happens and keep on top of things, rather than boosting a solo 10 seconds in and everyone noticing. also saves your gear if you can catch that loud scream before the otherwise quiet as fuck singer goes for it
I also second that people who try and tell you your job can fuck off. Without knowing the venue, band and having been present at sound check they've no idea what issues you're currently dealing with- especially in regards to vocals. 9/10 times if you walk into a venue and the vocals are buried, it's not the tech thinking thats how they should sound, its the crappy technique of the vocalist causing feedback at stupid low levels so telling him the vocals are quiet isn't likely to help.
Also, just a pet hate of mine. Turn off reverb and delay on the vocals between songs, sounds stupid as fuck when someones talking to the crowd and the effects are still on.
I love mixing live, Best job I've ever had. Metal is tough but since its what I started with I find more popular music so much easier. If you care about how the bands sound and try making it as good as possible it can be sooo rewarding. Those gigs where you get bands with good gear, who have gig experience who can play well are an absolute pleasure