My general goal is to take advantage of the stereo field to some degree, but hopefully not have the user particularly aware of it.

As you note, bass is omnidirectional. As a rule then, it's placed in the center. If nothing else, it stays out of the way instruments that you want to be heard in a stereo field.

Main vocals also go into the center, because putting them into the stereo field is just distracting. Panning for harmonies depends on how much I want the user to notice them.

Keys and guitars will be placed on opposite sides, as will similar combinations so they are balanced and the user doesn't hear more in one ear than the other.

Lead instruments will be placed slightly off-center to held separate them from vocals.

Of course, I'm willing to break these "rules" as I feel fit. laugh

In addition to panning, I like to add some sort of room modeling. I've used a lot of different VSTs for this, but the free version of Panagement remains one of my favorites. It seems to do something really nice to sounds.

That said, I don't think that panning is a very effective way to get everything to be heard. As Herb suggested, each instrument lives in a particular place in the spectrum, so using EQ to ensure that each stays there is a good idea. That is, cut frequencies above and below for each instrument that doesn't need to be heard. It's often suggested to have a low-pass filter on instruments to keep low-frequency mud from accumulating.

If there are instruments that are stepping on each other in similar frequencies, the most effective thing to do is figure out which is more important, and get rid of the other.

In cases where that can't be done - like where a string pad is encroaching on the intelligibility of the vocal - I use TrackSpacer to make space. Unlike a traditional ducker, it only lowers the portion of the audio where the frequencies overlap, so you don't get the "ducking" effect.

And as Herb said, using volume envelopes is great to bring in an instrument only where it's needed, and pushing it to the background (or removing it entirely) when it doesn't need to be noticed.

I think you can only expect a listener to pay attention to two things at a time, like the melody and a countermelody. A good mix makes it easy for the listener to focus on those one or two items, and keep everything else out of the way.

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?