Retaining a freelance photographer is a lot like retaining any other photographer, it will be very important to sit down and discuss their fees, and what you are willing and comfortable to pay. When you are working on a day by day basis they will like to see compensation as you go along. Pay them a small percentage up front to book the dates that you need them, than at the end of each day, pay them in milestones or per photo shoot. This allows you both to get work and pay in the same process, which is something that is important to both parties.