LOVE these tidbits from Mixerman....it could be pure genius at a momentary break in the song, and pure crap everywhere else."
"So if you bring up a part and it does nothing good for the track, provides no worthwhile counter-melody, serves only to distract the listener's focus from what's important, or offers no valuable rhythmic support, it's a stranded part, one that should have long ago been removed from the session. Underdub it."
"Since the vocal (or melody) is the most important part in a mix, you should make the preponderance of your tough decisions based on how a part affects the vocal"
"Remember, at all times, you're attempting to push the listener forward, manipulate their focus, and cause them to have an involuntary physical reaction to the music this will cause the listener to sing the song, which consequently will lead them to want to hear the track again."
from Zen and the Art of Mixing
Seriously, buy this book: