Break down the letter into short, but specific, paragraphs. Don't ramble, but do flesh out your key points. Try to keep it at one page, covering the various aspects of their recommendation in brief, specific detail.
Introduction
Your qualifications, how you know the candidate, general recommendation.
Professional Success
Why they will be effective, productive workers. Examples of successes.
Personal Testimony
Why they will be a great addition to work and office culture.
Closing.
Reaffirm your willingness to recommend them, offer contact information.