Be sure to not provide a full proposal of Service Prior to initial payment. Why? Unwittingly detailed proposals often give a prospect the "how-to" implement your service, which they are free to share with a lower cost (and often less experienced) firm or execute your service themselves. A scenario that happened to a 90's tech client when they thought they'd landed a whale.
Instead, provide a summary of service which highlights what you will implement but without the detailed instructions.
Don't let non-starters derail you. In a perfect world prospects will detail why they didn't hire you, but often they won't.