Here is a great page with a boatload of articles on mixing (among other subjects):
http://www.studiocovers.com/articles12.htm
The best analogy I've heard is the comparison of mixing to creating a landscape painting. I believe this analogy was made (in detail) in an article on the page listed above.
Essentially, close your eyes and listen. The goal is is to "spread the frequencies" around - while maintaining some semblance of aesthetic order - in a nutshell. You don't want all of your highs, lows, and mids all blaring away straight down the middle. Here are some of the things I try to do:
- If you can separate the drum tracks, do it. Arrange them as if you were sitting behind the kit. Pan the kick down the middle, snare as well (or perhaps slightly off center to the left). Pan the highhats off to the left - maybe even hard left - rides off to the right - cymbals where appropriate. If you can't separate the drum tracks (i.e. you just have 1 or 2 tracks), you'll have to experiment with panning to get the best possible sound.
- Pan the bass guitar dead center - it along with the kick drum will provided a consistent groove
- Play around with panning guitars left and right - you definitely don't want these blaring right down the middle - it will compete against your main rhythym elements. Perhaps pan the ryhtym guitar to the left, say the 9 or 10 o'clock position. Pan the lead guitar opposite to the left, say the 2 or 3 o'clock position.
- Some people prefer vocals panned dead center, some don't. I say it depends on the song and what effect you are trying to obtain.
It is also important to point out that there is no "magic rule". Every project will be different, because every project sounds different. You may find yourself adjusting certain tracks, applying EQ or reverb, for example. This may have some effect on how you "spread" the mix around. After applying EQ, for example, you may find that you like the overall sound, but you need more "separation" in a certain frequency range (mud, as its called).
Coincidentally, "mud" is also a painting term (I'm a closet artist). You can end up with "mud" on your canvas if you mix too many colors together - you end up with a dull, lifeless blob of paint. Same thing with mixing - too much stuff in one place and you end up with a dull, lifeless sound.
Hope this helps.
Cheers.
Scott