Dear Mr. / Mrs. / Ms. (name of the concerned person),
The first part of the letter is about the concern of appreciation letter. Mention the details of the reason of appreciation without paying attention to any other topic. Always try to appreciate on behalf of full staff. Mention the benefits the person is going to avail but do not forget to tell about as to when that person is going to be rewarded.
The second section of the letter should include the excellent work done or targets achieved by the appreciated person. Discuss something about the small achievements of that person. Motivate him/ her to go ahead with excellent performance to set an example for others.
In third segment, leave the scope to the person to clear his doubts and queries.