It must have strength within the field of science to which it pertains. Present your expertise, commitment, and the capability to do what you propose. And you need to have a solid and well-written grant that out-competes others for funding. Do your homework, allow enough time to develop a solid proposal, and present your idea in a manner that conveys your enthusiasm so you get reviewers excited about your idea.
Having trouble getting started? Have a case of writers block? The Texas A&M Division of Research provides general and sponsor-specific advice along with some additional resources.
“The Buck Starts Here” is an excellent Chronicle of Higher Education article (February 21, 2005) on the do’s and don’ts of proposal writing.
Is your idea fundable? Be sure to read The “So What?” Factor.