Include any agreements or arrangements made. It will show that you really care about your proposal and will make it more likely that he remembers to respond to the letter in a timely manner.
Example: If you have any questions, I'm happy to meet with you at any time. References Letters.
I wish to take a three-week leave to take care of my mother back home since she has been sick for a couple of months now. Conclude With Valuable Information Conclude your letter with your name, email address and phone number and invite the reader to contact you with any questions regarding the letter or the proposal.
Research is also key: you must know exactly what you require, know what the impact on your company will be and be able to explain it in terms that your boss can understand. Note that a well-written approval letter may not get you anywhere — your recipient may decide that the proposal itself is the problem.