Dinner music is also a plus. Be careful to avoid being Upsold. An extra charge for any of the aforementioned services needs to be addressed. Background music, proper dress and conduct, use of the microphone, special entrance music and introduction to the dance floor are all a part of a DJ's responsibility to the customer, and should not be charged additionally. Delegate someone to be in charge of possible extra time, and make sure to clear any overtime with the venue supervisor.
Cheap DJs are usually just that.
Some DJs overcharge for their service. Customer research, therefore, is essential. Allow yourself to be critical. Listen to the person on the other end of the phone. If no preview is offered, simply contact the next company on your list.