Dear (insert first name of contact)
Because this is a direct letter there will only be two paragraphs in total, so it is important to get to the point immediately. State all of the points you need to make logically and concisely, and make sure not to skirt around the subject. The reader should be in no doubt of what you mean within the first couple of sentences and your first paragraph should be no longer than this.
Briefly go over the main point of your letter, and mention that you would like a response in the appropriate time frame. Follow this with a polite sentence stating that you are looking forward to hearing from them and make sure to thank them for taking the time to read your letter.