Most of my friends know that I've been attending a six week (six Saturdays) class on DJing at Scratch DJ Academy in NYC. Our class of 25 has several instructors, the most famous being J-Smoke, the famous battle DJ from the Muthafuckin Allies.
One of the topics we discussed this week was the idea of a mixed tape, and how that's your calling card. Most DJ's don't make money off them, but if it's good it will get you gigs.
An example he gave was the Grey Album in which DJ DangerMouse mixed the Beatles White Album with Jay-Z's Black Album. Totally sick. More in wikipedia about it.
As for the rest of the lesson: having mastered baby scratch and dropping on the one, we spent most of the time working on timing as a precursor to beat matching (lining the beats up between two identical records playing at the same speed).