Ask all your prospective disc jockeys for references. If they're professionals, they shouldn't mind. After all, you're hiring them as an employee, however short-term the contract. If a DJ won't give you any references, move on to the next possibility.

Don't be afraid to negotiate with your disc jockey for the fee; all they can say is no. Be aware that some events, like wedding receptions and corporate events, will demand a higher fee; so will events occurring on weekends and during certain months.

Not all disc jockeys are created equal: some like to talk more than others. This is fine if you want them to act as an emcee, but they don't need to blabber on constantly. This is one point you should definitely discuss with them before the event begins.