In the first paragraph, introduce yourself and your company in brief terms. Briefly mention the reason for your letter.
In the second paragraph, politely put forth the request and mention the reason as to why it has been written to him. Also mention the return of favor to the reader. Give alternatives to the request, if any. Do not apologize to asking the request. Ask for a meeting if the favor is too big or you need more explanation to do.
In the last paragraph, thank the reader for considering your letter and mention where he can contact you for further correspondence.