Sample letters of appreciation and gratitude for both personal and professional instances can be found online from several reputable sources including About.com. Regardless of the circumstances, the letter should include specifics, reasons and personalization to reflect sincerity.
A personalized thank you or letter of appreciation is handled similarly whether of a personal or professional nature. Begin by thanking the respondent for the specific action. Expound upon how that action impacted the business or personal time. If the letter does not pertain to a specific action, list reasons for gratefulness. Explain the significance of these reasons. Finish with a friendly sign off such as "Warm regards" or "Respectfully yours."