Read this great article on the art of being the opening DJ

Quoted from the Resident Advisor:  

 "Whereas a nervous promoter wants to see the dance floor packed with people pumping their fists in the air from the start, all the DJs I spoke with saw this as detrimental to the night as a whole. As Richards put it, "There can be nothing worse than an over-enthusiastic start—a soup that burns the mouth or curtains ripped open to let the light in."

The entire article should be necessary reading for all up and coming DJs, read it now over at Resident Advisor.