Sound Checks are for Sissies
I do a lot of events where the set change is set up at 30 minutes or less. This leaves no time for the traditional starting point of mixing a live show - the sound check. If I have time for a sound check the thing that I am most interested in is getting the input levels in the ball park. Even the most professional musicians seldom play the same during a sound check as they will play during the show with a crowd of fans in front of the stage. From my standpoint the primary purpose of the sound check is to get the musicians comfortable on stage. If everybody on stage can hear what they need to hear and what they want to hear it will be a good show.
When the Show Starts
Mixing a live show in a matter of taking care of the glaring problems and then fine tuning the mix. The first thing that you want to do is pull back or trim back those things that are too loud. One of the most difficult concepts to grasp in mixing sound is that probably half the moves you make are pulling back the things that are too loud.
Turn Up or Turn Down
If you are having trouble hearing something the decision that you have to make isthe reason that you are having trouble hearing that sound. It can be that the sound is not loud enough or it can be that something else in the mix is too loud and preventing you from hearing the sound. The relationships must be right to get the best mix.
A case in point could be the vocals. If you are having trouble getting the vocals out you must decide if the vocals are not loud enough or if the real problem is that the guitars (or keys or horns, depending on the instrumental compliment) are too loud, preventing you from hearing the vocals in the proper perspective.
The same thing can happen with the kick drum and the bass guitar. Is the reason that you are not hearing the foot because the bass (guitar) is too loud? This can happen at any place in the mix where frequencies of instruments overlap.
The Train of Decisions
So the mixing of a live show is a constant train of decisions as to what is too loud and what is not loud enough. With experience this will become almost an automatic reflex, but if you are just beginning to learn the art you will need to keep this in mind on a more conscious level.