Formal thank you letters end differently from casual or friendly letters in that they maintain an air of professionalism and cordiality, rather than the warmth or humor associated with less formal letters. You might find occasion for this when writing a thank you letter to a potential employer, a teacher or to someone who you do not know well. Closing the letter formally ensures that you do not assume the colloquial tone associated with close friends, which might appear disrespectful.
Formulate the closing paragraph. This last paragraph often begins with a restatement of the writer's gratitude. Include in this paragraph the reason you're writing the thank you letter. For example, job interview thank you letters might include a sentence that says, "Thank you again for your interest in me as a potential candidate for employment.
Tell the recipient you look forward to hearing from or seeing him again soon. Include phrases such as, "I am looking forward to hearing from you soon," or "I look forward to receiving your decision." To make the letter sound less canned, Yale University suggests using words such as "I am excited to gain more knowledge about ..." or "This position sounds like an exciting opportunity." Such sentences work well as a closing statement.
Finish with a formal closing line, followed by your signature. "Sincerely," "Sincerely yours," "Respectfully yours" and "Yours sincerely" are all appropriate closing phrases.