There really is no formula. The idea is to do what it takes to make it sound good. And if it already sounds good, to have the self-discipline to leave well enough alone.
EQ is usually something you only want to add to fix a problem. If a track doesn't need it, that means you are better off. In general, subtractive EQ is preferable to additive.
Panning is a matter of personal preference. Some people like to create really wide soundstages that wouldn't necessarily reflect a natural listening experience (e.g. 15 foot wide pianos or drum kits). Other people prefer panning that creates a more natural spread. There's no right or wrong, just personal preference on a given song.
Panning can also be a tool to create seperation between competing elements (multiple comping instruments, e.g.) It also is a way to get background elements out of the way of the lead vocal, kick, bass, or snare (which are traditionally panned down the middle). Again, it still comes down to what a particular song needs to sound good.
Experimentation is your best bet. Take background vocals for instance. See how wide vs. tight panning changes how they work in the song. Then try experimenting with rolling off the low end. A little bit might open up the mix, but too much starts to turn them into a "special effect" (which might be good or bad, depending on the material.)
