The creative process starts with the key message in the communications brief.
The key message is the single (or fewest possible) main thought(s) that you want the communication to convey. Ideally no more than one sentence long, it's the one idea that you want the customer to grasp.
Avoid unduly influencing the thought processes of creative suppliers by using creative language to express the key message. Instead, describe it in purely rational terms.
A set of words that present the customer with your key message in a simple, involving and sometimes wholly unexpected way.
The proposition is something a copywriter delivers. Think of it as the title of a sales brochure, the headline to an advertisement, an exhibition panel, the line at the top of a web page or the front of a piece of direct mail.
The surprising, simple and memorable creative approach to communicating the proposition. The best concepts take the proposition as the starting point and carry the thought forward.
NB: If the concept can be described, understood and accepted without the use of visuals, you know it's spot on.
The way the agency expresses the concept in the appropriate medium.
It's essential to separate the key message from the proposition, the proposition from the concept, and the concept from the execution.
This will leave you free to comment on the qualities of an execution without revisiting the concept.
You should also get more concepts to evaluate and therefore stand more chance of finding the execution that will work across a variety of media.
Last but not least, you'll be avoiding the petty jealousies that are bound to arise when several agencies are working on the same project and only one execution goes forward.