This is where we separate the men from the boys!
At a party it's often the case, the DJ's primary objective is simply get 'em up and keep 'em up. Frequently the mobile DJ is faced with all ages of folks with a range of musical preferences a mile wide. A disc jockey can have the best equipment and years of experience, he can be a member in numerous professional associations and have the largest music library on the face of the earth, BUT when the smoke clears, it always comes down to the same thing: what are ya gonna play and when are ya gonna play it!? Sure good mixing skills and the ability to do and say the right things to motivate the party goers contribute to more participation on the dance floor, but the real science and art to packing the floor and keeping it packed is PROGRAMMING. WHAT ARE YOU GOING TO PLAY NEXT!? The common technique descriptions are: "reading the crowd," and "playing the crowd." Approaches vary per DJ and per event. The key to opening the door to getting folks up and dancing is: providing the grounds for attendees to allow themselves to participate and have a great time. This is easily the material for a book with many chapters. A disc jockey's worth is OFTEN measured (and quite rightfully so) by how well he accomplishes cooking up a packed dance floor and keeping it percolating.
(The above scene from a wedding at the Crown Reef in Myrtle Beach features an "average" group of guests most of whom do not frequent night clubs nor ever have repeated opportunities to dance. Tapping into their receptivity to dance through friendly encouragement, warm invitations, and an effective presentation of the right kind of dance music calculated to appeal to their varied inclinations, and built upon their cumulative response, yielded this unpretentious dance floor scene where those who participated were not self-conscious, but just had fun as part of the wedding festivities. It was a great party.
Here's an informative article on the subject by WED Guild™ member Larry Green.