Generally the best method would be to bring up the loudest sources in the overall mix so that they just go into the yellow on the channel (on their strongest parts of the song) bar graph and then blend in the other sources relative to where they will end up in the mix.  Now bring up the mains until you get the level you need for the venue (or until you run out of system)

You will want to do this with an eye to being somewhat equal between the channel absolute levels and the mains absolute levels.  In otherwords don't run the channels up into the red and the mains way down low in the green.  In this case bring each input channel down a bit and then the mains up a bit.  Ideally you should be almost equal but with a little morre gain on the input channels rather than the mains master.

If you find yourself exceeding the capabilities of the system( i.o.w. ... you're way into the red and still not loud enough in the house)  before you have enough level in the house take a look at some of the presets available for the mains outputs.  There are some "loudness" presets that can raise the average level of the system even higher before completely running into the hard limits.  You give up a little "dynamics" but gain overall loudness.  I would not regularly use them unless you really need them, but if you like the way they sound then go ahead.