Like @rdon stated, lawyers cost a lot, and that's fine if you have the capital. But if you are in the early stages of a start-up without the resources, all you need is a written document that states your terms that you would liked 'formalized', make a copy of it (prevents people from conveniently misplacing it), then both sign it with dates (consider having a witness sign it too). Then in the event that you need to 'sort' things out, both your pieces of the signed document act as your contact.