Once you have enough information to actually write the letter, set aside anywhere from an hour or two to write it. Remember that, if the applicant needs five versions of the letter (say, for five different graduate programs), you don't necessarily need to write five separate letters. Application forms usually request the same type of information, and you simply need to create variations based on a template.

MAKE A LIST

A great first step is to jot down a list of keywords and key phrases -- accomplishments and qualities -- you want to address within the letter. Such a list will help you avoid the most common mistake made in these letters: vague, metaphoric writing. Anchor your writing to specific images and anecdotes; provide concrete, convincing evidence. 

Classify your keywords into: 

• professional/academic qualities 

Is this person a leader?
Is this person a strong team player? 
Is this person a keen analytical thinker?
Is this person capable of conducting sophisticated research?

• specific skills

Strong communication skills?
Fluency in certain languages?

• personal qualities 

Does this person get along with peers? 
Does this person have a good sense of humor?

• past accomplishments 

What are the principal two or three accomplishments this person has achieved under my guidance?

• weaknesses

What are some points this person must address to advance his/her career?
How well does this person learn from past mistakes?

• future potential 

What do I believe this person is capable of achieving?

Letters of recommendation vary widely in content and form, but solid letters contain a number of common traits. Here below is an outline for a well-structured letter that covers all main points. 

INTRODUCTION

First Paragraph: Introduce Yourself
Begin the letter by clearly stating your position, where you work, your relationship to the applicant, and how long you have known and/or worked with the applicant. 

Second Paragraph: Give Your General Impression of the Applicant

BODY

Third Paragraph: Applicant Quality # 1
State the applicant's most noteworthy quality, and support that claim with a specific anecdote. For instance, you might say that Samantha is, first and foremost, a born leader; then, support that statement by telling about the time Samantha took the initiative to form a task force to deal with a glitch in the company's computer accounting system. 

Fourth Paragraph: Applicant Quality # 2
Again, state your claim with specific anecdotes and concrete examples.

CONCLUSION 

Sixth Paragraph: State why you think the applicant's plans suit him/her.

Seventh Paragraph: State how you think the applicant will contribute to the program or company. 

Last Paragraph: Strongly reaffirm your confidence in the applicant's abilities and conclude by telling the readers they should feel free to contact you in case they need more information. Don't forget to include your contact information beneath your signature and name. 

NOTES ABOUT THE FORMAT AND LENGTH: 

Don't handwrite the letter; type it. Handwriting a letter is a sign that you are not serious about the task and will reflect poorly on the applicant.

Remember to use official letterhead, to sign the letter, and to include both complete contact information. When you have folded the letter and put it in an envelope, sign across the seal. 

The length of letters of recommendation varies greatly, but five paragraphs is usually the minimum. On the same note, don't go overboard and churn out seven pages, even if you are highly enthusiastic about the candidate. Choose your content wisely, and remember that a concise letter is usually more effective than an overly verbose one.