Consider sending a list of your board members and their professional affiliations. It is possible that the celebrity is affiliated with one of them or is acquainted with their company. If the celebrity has something personal to gain from helping NAF, it is appropriate to say so. Based upon our experience, we can expect x number of major national magazines to publish a full-page version of the ad, bringing public recognition for your generous contributions to the Academy. Do not allow this process to intimidate you.
Brevity is the soul of wit.
Your letters should be edited down to the essential information. Many readers are not inclined to read beyond a page of text, so distill your request to a single page. In addition, allow a few other people read and react to your letter before you send it off. You'll be surprised at the ideas that others may have, or the way people interpret what you thought you had expressed clearly. Mail your letter once you have incorporated input from your staff.