Begin the letter by addressing it appropriately to the recipient, and not with a generic "Dear Sir or Dear Ma'am." Your letter is a lot more likely to be read if you use the full title of the individual to whom you're sending it; i.e. "Dear Congressman Smith." Do not use first names, unless you know the recipient personally.
State the reason that you're writing the letter right away in the first paragraph. Doing this saves the reader valuable time and is more likely to immediately grab his attention; i.e. "Our nation is in dire need of funding for economic development. Our infrastructure is crumbling around us and something must be done." Remember to keep it brief, concise and substantive. The reader will appreciate your ability to articulate your thoughts.
Present the big idea in the next paragraph. Explain to the reader how you think the nation can get funds for economic development. Give enough information to pique their interest, but not so much they get lost in the details. Provide sources for any information you cite that's not common knowledge and refer the reader to additional reference sites if needed. For example, "While the State of Montana is in the top ten for receiving federal funding for economic development, I feel that we are missing many other private and government agency funding opportunities..."
Reiterate the reason that you're writing and what your big idea is in a sentence or two in your closing paragraph. This reinforces why you're writing and it gives you an opportunity to briefly state why you think your big idea is worthy of consideration. For example, "Our nation's crumbling infrastructure is proof of our immediate need for greater funding for economic development. By pursuing other private and government agency funding dollars we can immediately halt this decline and slowly begin to rebuild our nation's infrastructure. This model has been used successfully in Canada and there's no reason it shouldn't work in the United States."
Close the letter by thanking the reader for their time and consideration. If you plan to follow up with the reader be sure and include the specifics of when they should expect to hear from you; i.e. "Thank you for your thoughtful review and consideration of my idea for garnering more funding for economic development. I will contact you next Friday, April 8th, at 10 a.m. to discuss this further." Proofread your letter and ensure that you've followed correct business formatting before sending.