Ok, I've been spinning for three years now and have studied harmonic mixing to great lengths. I understand the principal of chords, Chord progression, etc.
I feel I may have done myself a slight diservice in my endeavors because, when it comes to mixing live, I still feel I'm not quite hitting the mark.
What I want to know is how do you mix harmonically in a live situation? I've boilded it down to just selecting your track by feel as you go and once that track is selected, applying harmonic pricipals to the way you mix in that selection.
How do you balance Harmonics with track selection in a live setting and, for those superstar DJ's, take me through your process of selection while mixing.
For example, do you hum the melody of the records you shuffle through as your selecting? What makes you stop and say: "This is the perfect one!" What's that process? WHERE IS THE HOLY F'n GRAIL!?!

Have you ever played live?
Not to be judgemental but it doesn't sound like it from the way you come across. ( disregard my comment if you've done many club gigs already )

I think there is no given answer to your question; practice makes perfect, hard work is the key.