JobSearch.About.com has a good example of a formal request for a leave of absence letter. The letter begins with the city, state, ZIP code and phone number of the employee making the request. Next is the name of the supervisor, their title and the organization's address.
The next line of the letter is "Dear" plus the name of the supervisor. Then the person making the request says that this letter is a formal request for a leave of absence followed by a description of the specific dates being requested off and the exact day the employee will return to work. The employee ends the letter by thanking the supervisor for considering the request for a leave of absence and asking to be contacted with any questions.