i think the hardcore mix i put on your hardcore thread was mixed in key. it's easy to do. i use a program called mixmeister6 to find the key of my tunes. (you will ave to either record your records to mp3 or download the mp3). once you have the key, everything else falls into place. for every 3% pitch change, the key changes by half an octave. so a tune that is too fast/slow and a couple of keys out could still fit into the mix.
it can fry your melon if you let it, but when it works, it's seamless.
this is the key chart. it works either direction but only one square at a time (no diagonals).
Example; you're playing a song that is D-flat major(3B) for key mixing, your next tune should be either a 2b,3b,4b or 3a. once you get you're head round it, it'll be the first thing you do when putting a set in order.