When I'm mixing it's a lot of fun to layer tracks and keep them together in the mix for an extended period of time. This is usually pretty easy to do with techier tracks where chord changes aren't a strong component, but it's much more rewarding when I stumble on a combination of more melody/chord-oriented tracks that complement each other perfectly in the mix (because they're in the same key). I discovered one such combination several weeks ago when I was doing my radio show, moving into more of a progressive breaks set to finish out the show; I was playing "F.L.E.D." by Ben & Lex and had happened to choose the DJ Hyper remix of the Paul van Dyk single "Crush" to play next, and noticed to my delight while I had it cued up in my headphones that the keys matched perfectly, so that I could bring the track in a little earlier and layer them in the mix a little more rather than taking the approach of just chaining them together.