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.
Some applications, such as professional schools give applicants the choice to waive their right to view their letters or not. Usually, applicants are encouraged to waive their rights because if they do not, it is a sign they are not confident in their recommenders.