Make comparisons to illustrate their success. To put the candidate’s accomplishments into perspective, include comparisons so that the recipient will have some basis to understand why you are recommending this person.
Target the recommendation. Is it for an academic post, a job, a volunteer position, a background check, or a personal reference? Write your letter so that it is focused on this purpose. For example, if the letter is part of a job application package, it should focus on the professional qualifications and conduct of the candidate.