Keep your writing action-oriented. Begin each paragraph with a punchy, active affirmation of the candidate’s qualities or character. Strong verbs are your friend.
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.