Stick with the facts and avoid unsupported assumptions. Present enough evidence to support your position, and no more. Pages of charts and graphs will probably not be read, and often fail to support the thesis of the writer. To test the clarity of your writing, have friends and family read your proposal. Some of your best comments will come from people unfamiliar with your field, not operating at the same assumptions as you, and unfamiliar with your jargon.
Vigorous writing is concise.
A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. In language, clarity is everything. To do this you need to describe what you want to do, how you plan to do it, and why you're best capable of doing it. Note that this reiterates what is in the proposal. The cover letter should duplicate the information in very brief form.