Choose Your Format
In a formal situation, when you're writing to thank someone for giving you a job interview, making a donation or entering into a contract with you, use proper business-letter format with an inside address, date and courtesy title in the salutation. Stick to a modified-block format, in which you indent the first line of each paragraph about an inch. If you want to thank a customer, this is one business situation in which a handwritten letter is not just acceptable, but impressive in these rushed and automated times. If you're handwriting your letter, use blue or black ink.
Open Your Thank-You Letter
Open with a friendly salutation, and address your recipient by courtesy title and name. "Dear" or "Greetings" or "Good day" all work. Then get straight to the point: "Thank you for taking the time to read my proposal." "Thank you for your generous donation to the Feral Cats Fund." "Thank you for your purchase of an efficient gas-fired furnace from Bob's Reliable Heating." Your second sentence expands briefly on how pleased you are: "I know your time is valuable, and appreciate your considering my work." "Donations like yours make it possible for us to come closer to our goal of solving the community's feral cat problem." "I think you'll be very happy with the state-of-the-art technology and lower heating bills."
Paint a Picture
In your second paragraph, fill in more detail about the reasons you're expressing your gratitude. If you're thanking the recipient for an interview or admission to a program or group, it's OK to use this space to restate why you're qualified and what you look forward to contributing. Mention specific points that arose during the interview or follow-up questions that arose. If you're thanking a customer, you can remind him of the benefits of your product or service. If you're thanking someone for a donation or a scholarship, let her know that the money will help spay 10 feral cats, or that you will make excellent use of your opportunity and a little about what that means to you.
Thanks Again, and Goodbye
Finally, thank the recipient one more time. "I very much appreciate (whatever)." If you are committing to do something in return, or encouraging the recipient to take the next step, restate this action briefly, as well; include your contact information. If you're thanking the reader for an interview, briefly restate why you're an ideal fit for the job and your eagerness to work together. Close with "Sincerely" or "Best regards," and be sure to sign the letter in ink above your typed signature. Read your letter aloud to check for sentence structure and overall flow, and edit carefully for typos and misspellings.