The bride is the most important person on the dance floor
If she's not feeling it, you're doing something wrong. If she is feeling it, you're immune to any criticism.
Inviting guests up during the parents' dances stacks the deck
Three minutes of father/daughter dancing can seem like an eternity. A pro DJ move is to ask the bride's permission to invite others onto the dance floor toward the end of the song. It cuts the tension, but more importantly gives the DJ a more crowded dance floor with which to get the action started.
Opening with a fast song isn't a good idea
After the formal dances, it's intuitive to start the party with a banger, but no matter what you play, no one wants to be first to a dance floor. Along the same lines of the parents' song gambit, the pro move is to open with a slow dance to prime the floor.
Slow songs are a reset button
Slow songs work to freshen the crowd if a floor is losing momentum, as someone's wife or girlfriend will always force them to dance against their will to "Unchained Melody."