Once you have designed your business and defined your prototype offering and a 1st strike prototype plan, you need to pull together the team that will help you develop the prototype.
Make sure that the team is as small and cheap as is physically possible. And then divide the cost by half. If at all possible try and develop the prototype yourself. If it is a Web based prototype there is little reason today, given all the free Web 2.0 tools out there, why you should not be able to develop a prototype alone at almost not cost. Research the tools to death to ensure that you use the right ones in the right way.
If you need to develop a physical product prototype for your business, then try and mould, cut or shape a rough version yourself first before considering taking it to any kind of a design firm. Start with sketches.
If it is a services based offering that you are developing then you definitely can develop the prototype yourself. First you should design the service on a large piece of paper using simple process design. Then document it. Then develop it as a working digital simulation on your computer.
Only hire a team or design firm to develop the prototype if it is absolutely essential and you can not do it yourself (or with your friends and family). If you do need to hire a prototype development team make sure that they have EXACTLY the right skills and experiences. They need to be a round peg into the round hole of your prototype. And cut the best possible deal possible when it comes to payment. See if they will take equity or part equity for payment - this is your first major deal since setting off on your venture - make sure it is a damned good deal.