To be effective, recommendation and reference letters must be insightful and personal. However, they are by nature quite formulaic. Most consist of three sections (the opening, body and closing), and rarely exceed one or two pages in length. Reference letters of fewer than 200 words are on the short side, while letters of more than 700 words is getting a little too long. Remember, the recipient probably has many such letters to read and review in the course of his or her job.
Employment and academic recommendation letters are written in business "block" style, with no indents and two spaces between each paragraph. Character letters and other personal references can be formatted that way, or in traditional letter format, with indents and no spaces between the paragraphs.
Some recommendation letters, particularly in academic settings, follow a set format. The requester might even provide a fill-in-the-blanks style form.
After your return address and (if appropriate) the recipient's address comes the salutation. If you know the name of the person who will be receiving the letter, by all means include it, along with the honorific Mr., Ms., Dr., etc. and a comma. In other cases you should just go with To Whom It May Concern, followed by a colon.
The opening or introduction takes up a couple of sentences or the entire first paragraph. This is where the writer establishes why he or she is writing the letter: to recommend someone for something. If you're writing a reference letter, include your relationship with the person (project supervisor, academic adviser, friend, parole officer, etc.), and make it clear that you are in a position to judge his or her performance. It should be made immediately clear what your opinion is of the person you're recommending.
The body of the letter will be its longest part, taking up several paragraphs. (At least two but probably no more than five.) This is where you describe the person, hitting on relevant character traits and skills. It's also good to include an anecdote (short story) or two that illustrates these attributes or knowledge. Describe the person's qualifications and qualities using descriptive adjectives, but be sure to back it up with concrete examples.
In the closing paragraph, wrap things up by reiterating your opinion of the person (presumably you hold them in high regard). If it's an employment reference, perhaps state directly that the person is a good fit for the job. Many letters end with a line offering to provide more information, and provide a phone number and/or email address for this purpose.
The letter should be signed personally by its writer.
Letters of recommendation should rarely, if ever, be handwritten. Produce the letter via computer, and save it for future reference. One letter can serve as a template for subsequent revisions, or even adapted to suit later requests by other people (Although, like re-gifting, it's important to keep track of who gets what letter.)
Index of letter of recommendation templates