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.
Familiarize yourself with the position. Get a copy of the job description, if you can, and talk to the person you will recommend. If you know the intended recipient of the letter, ask them about the position as well. The more you know about the position, the better you will be able to tailor your letter to suit the needs of both parties.