A contract is important when booking your wedding music. It should clearly indicate the cost, the times that the music will be provided as well as any additional costs you may need to pay. In addition the contract should indicate the cancellation policy and refund amounts for cancellation.
If you are using a single DJ or a band, always have a plan for how to cover the entertainment should they cancel. If you are going through a larger DJ company they will typically have additional DJ's on staff that can fill in, but a single DJ may not have that option.