DJ's need to have their finger on the pulse.
Track selection is important - It doesn't matter if u can mix if the tunes aren't good in the first place!!!

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"I expect to pass through life but once. If therefore, there be any kindness I can show, or any good thing I can do to any fellow being, let me do it now, and not defer or neglect it, as I shall not pass this way again."

In my opinion it also helps if the DJ can play a little with the songs in the mix during the transition (Eq stuff, switching bass of different records in and out of the mix, mid-sweep-ish things)