Clients Need to Assume Ownership of Their Projects

It seems as if there are many intelligent, well meaning people in this world that have a great concept or idea they want to bring to market.  It's always the same pitch:

We have an idea for Whiz-Bang 1.0 that will completely revolutionize the current market.  We are the experts in our market, and we know what we need.  Once you develop the product, we can make millions, millions I say

So ok, we as developers buy into this revolutionary concept and try to bring the concept into reality.  Now keep in mind, these people that approach us as developers are usually in a sales role, or they may have enough technical knowledge to be dangerous, but they never can quite quantify that the person that they just hired on has absolutely zero concept or clue as to exactly what is required to deliver.

So, as a well meaning software development professional, the first step of the SDLC is to always define a set of requirements or some sort of contract (albeit nothing enforceable by a court of law, but still...) that sets down on paper the goals of the project, in as much business detail as possible.

Now on multiple projects back-to-back, I feel as if I need to come up with a better mechanism to handle this.