Buy a blank card or sheet of stationery that complements the message you will be writing. If you're writing a message of love, choose stationery with hearts on it.
Address the letter recipient appropriately. If you're writing to a long-time friend, use his name. If you're writing to a boss or other respected member of the community, use "Mr.," "Mrs." or "Ms." If you are writing to profess your love, use a term of endearment such as, "My darling," or "My dear."
State the reason you are writing the letter. It is effective to immediately explain why you're writing, rather than delay with small talk or pleasantries. If you're writing to express your appreciation to a friend, begin with, "I'm writing to you today to tell you just how much your friendship means to me."
Provide specific examples to support the message of your letter. For example, if you're writing to express your thanks for a close friendship, explain how your friend was there for you during a difficult time in your life, and how much fun you have when you are together. Expand on those points by providing memories of how you felt during the times in question.
Compliment the person, if applicable. If you're expressing your love, write a paragraph listing all the qualities you love about the other person. These qualities could include factors such as her appearance, intelligence, attitude and outlook on life.
Conclude by reiterating the reason for the letter. For example, you can say, "We get so busy that I sometimes forget to tell you how much our friendship means. Please know that even when I don't say it, I truly value you as my best friend." Sign off with a term of endearment such as "Your best friend," if applicable, and sign your name.