"My advice [to parties contemplating having an LOI] is to proceed with caution before signing any such document. As a general rule (and there are exceptions), I urge the parties to go right to the final documents and "dot all of the I's and cross all of the T's," rather than go through this interim step of a letter of intent, which has many potential traps."
At the end you should sign off with "sincerely yours" or any other related salutation. If you are writing a letter of intent for graduate school then it would have a different content. First of all letter of intent would start with your name, address, contact number and email address. After you have mentioned the date, you should then add a salutation like "Dear Sir/Mam".