My number one mixing goal....stay in control. Knowing how to eq compress and shape a mix is all dependent on keeping control of the signals that are combining. This is the foundation of a good mix. If your signals are out of control you will spend your time chasing the "mix" and no time to artisticlly shape the sound.
Here's how I was taught. First learn what a decibel is as it relates to electronics and acoustics. Now according to the voltages of an audio signal in a mixing console the use of a simple formula can aid in setting signal levels to keep things under control. Here's the formula.
For every doubling of channels used back each channel down by 3 decibels. That's it. Memorize it. You will use it every time you mix.
So to put it into practice here's how it works. If I have two singers singing a duet I would tell the first singer to sing and with the master fader on 0 and the channel fader on 0 I will use the trim/gain knob to set the level on the meter to -3. I'll do the same with the second singer. Now when they both sing the channel voltages will combine and sum back up to 0 on the board. Now let's double the channels to 4 channels being used. 2 singers a keyboard and guitar. Go through the same process of setting up the channels one at a time. Each channel should be set to what? Minus 6 db. Now when they all play and sing together the output sum will be 0. Double it to 8 and each channel should be set up to minis 9. 16 channels being used should be set up to minus 12 etc. etc.
Remember this is to get you started. If you change the eq it will change the level of the whole channel. Keep that in mind. There are other factors that need to be taken into concideration like how we perceive sound. I'll get into that in another blog.