When complimenting your boss in writing, be as genuine as possible. If you aren't specific, the compliment may not be as effective; if you exaggerate or are too "over-the-top," then it comes across as phony or "brown-nosing." In your complimentary letter about your boss, stick to the facts and explain why her actions/words impressed you enough to draft a letter about it.
Introduce yourself in the letter and clearly identify your relationship to your boss. In addition to your name, state your position and how long you've been with the company. Also, explain how long you've worked with your current supervisor and the kind of working relationship you share.
It is insufficient to simply write, "He is a great boss." Illustrate and point out the reasons for the compliment. For example, you might say "Mike is a great supervisor because he allows employees to have a flexible schedule, he offers weekly meetings to learn how we can improve and he regularly offers constructive criticism." The more specific you are, the more effective the compliment will be.
Your boss has likely done several things about which you can compliment her. However, when writing a letter, stick with the specific action that spurred the correspondence. Mention some of the other things that you like about your boss, but focus the letter on the specific action that you are complimenting, such as "Michelle specifically intervened on a project that we were struggling with and came up with a solution to save the project and help it come in on deadline."
Close the letter by explaining why you felt it important for your boss' good deeds to be recognized. Mention that you just wanted to be sure he was recognized for his positive supervisory direction. Alternatively, write the letter to nominate him for a company-wide prize or recognition. Say "Thank you" for taking the time to listen to your positive feedback about your boss and wish the reader a good day.