Here are my most important mixing rules.
Take mixing one step at a time
Understand what you want to do and why you are doing it. If you a solving a problem in the mix, understand what that problem is. Take care of one thing at a time.
I just spent a day with a guitar track that had too much sustain and was dry and muddy. I got rid of the sustain, checked the mid to low end, added sizable reverb, and tried different panning. None of that worked. I realized that the only thing bothering me was a sustained ringing around 3.5 kHz. I added a slight EQ (/wiki/equalizer) notch around that frequency. This allowed a more interesting reverb. The sustain no longer bothered me, so compressors were not necessary. With the new reverb, the low end was not as much of an issue. I tried many things, until I understood that I only needed a slight EQ.