There isn't an easy way to do it. If there was, I'd be out of a job!!

The golden rule is 'keep the loud instruments quiet enough so that you can hear the quiet instruments'!!

In other words, once you've got the quieter instruments to the point where they start to 'ring' (onset of feedback), you won't get them any louder, so the louder instruments have to be turned down to balance up.

Does that help? Thought not... but I tried!!

Seriously, try headphones to monitor the mix output - that may help - but don't rely on on-stage foldback monitors, they give the wrong 'feel'.