Compression/Sustain. How does everybody use it? Do you use it on all songs? Or dialing in a varying degree depending on the expression you are trying to achieve?

I know what is does. Compression takes the amount of attack and averages out the volume, while sustain allows the note or chord to hang a round a few milliseconds longer.

I have no idea on how to dial in the amount needed to do any thing. I have read that too much is an over kill. Can someone explain what a little vs. a lot will do to a note or chord?

Thanks!