Make sure you have connected everything properly and turned your power amp all the way down. Your gain level won't mean anything if all of your equipment is not properly plugged in, so verify your inputs and outputs.
Set your channel faders at unity and your master fader at unity or a little below. It is tempting to rely on faders for most of your gain, but this will not give you the optimum signal to noise ratio, if you can get signal at all. Set your channel faders to unity on your board (usually 0dB). You will use the faders to adjust the final mix but they should be at unity for getting levels.
Turn up the gain at the preamp until you see a clip, then turn it down 5 to 10 percent. Your preamp gain is the place you want to get most of your signal from. If your preamp is clipping, turning down the fader won't fix it. To get a good setting, turn the gain up until it is barely clipping and then turn it down a healthy little bit. This will give you a good amount of signal with some headroom.
Adjust gain on a compressor or other processing unit instead of the mixer when using external processing. If you turn up the gain when running through a compressor, you will send more signal to the compressor but not necessarily get more volume from the output of the compressor.
Increase the volume on the power amplifier until the room is comfortable sounding. This may mean cranking the volume on the power amplifier to fill the room, or you may only have to turn it about half way. This should give you the best signal to noise ratio.