most important point: stay calm!
don't go crazy with the EQ during the line check. if you don't have much experience in live sound chances are what you're doing to the instruments EQ wise during line check won't work in the full mix anyways....i'd just try to get rid of the shit that you KNOW you won't need in the mix, like any excessive low or high end etc.
watch your gain staging.
it's a good idea to compress and gate the drums. compression on the bass DI also works nicely.
don't overdo the fx stuff. i tend to set up a subtle reverb on the snare, and a quarter delay for vocals. use tap tempo for the delay and ride it during appropiate sections.
in terms of monitor mix, less is more. if the band is constantly complaining that they can't instrument X on stage it's likely that instrument Y is just too loud on stage.
go out there into the crowd and check how the mix is sounding there. how things are sounding to the majority of the audience is what's important, not the sound at your mixing position.
personally, i never hard pan anything. a lot of the times you can even get away with mono. try panning the guitars out like 20 % L/R to add some seperation, pan out the toms like 50% L/R and that's it.
