To write a letter to thank someone for supporting you, it is important that you express your gratitude sincerely, eloquently and briefly. Although it is tempting to write a long letter with the assumption that doing so expresses your gratitude more profoundly, rely on the principle that shorter is better when it comes to letters. Focus on the quality of what you say rather than trying to create a greater quantity of words.
To properly express your appreciation for the support you have been given, start by thanking your supporter directly and succinctly. Be specific as to what you are thanking the person for; make sure that you do not write so generically that your expression of thanks feels like a form letter. Make a statement about how the support you have received has made a difference in your life, and give specifics. Make this section of your letter as personal as possible, depending on your relationship with your supporter. Close your letter with a repetition of your thanks, phrased differently than your initial expression of gratitude in the opening. Be sincere throughout your letter. Depending on your relationship and situation, your letter may be handwritten, typed or printed. Proofread your final letter to keep it error-free.