If you can find it out on the web Cap Capello was on a podcast regarding contracts. While Cap isn't a lawyer he is the next best thing on line but as he would tell you, seek the advise of a local attorney.
Cap has had some great contract posts here also. Cap also uses a very unique approach to booking events. Don't book the service, book the date, after all, your calendar of dates are your inventory and you will only have x amount of inventory(time) to sell. Once it's gone it's gone.
IF someone buys a date from you and then backs out say at 30 days or less til event, what are you're chances of rebooking? You now have spoiled inventory. You will stand a better chance of collecting your cancellation money, because you sold a date not a service. Of course how you have the contract drawn up will make all the difference.
As Cap says, the contract is to keep you out of court in the first place.