It depends on the gig and the situation. For weddings, its different than for clubs. It would be different if it were an mainstream club or a genre specific club. This is where the experience part of DJing comes in at.
For social events with a meal like weddings. I may either do something interactive like a line dance to get people up. Maybe play a classic slow song and announce the dance floor is open and then play a fast song.
For a multi-genre nightclub, I will usually stop the music and make some sort of house announcement, drink special and/or welcome if its early in the night, and then switch the genre from what I was playing to an anthem of a new genre and that usually works well.
For a genre specific club, its more subtle. I will usually open with a lot of older but recognizable tracks and then slowly start to bring in newer tracks that pull people out on the dance floor.
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