Start by identifying your relationship with the person for whom you are writing the reference letter.
State how long have you have known or worked with the person.
Point out a variety of positive traits while focusing on work ethics, accomplishments, skills, and significant contributions (use specific examples).
Avoid vague, powerless words such as nice, good, fine and reasonable.
Use words such as excellent, superior, instrumental, creative, innovative, efficient, dependable, articulate, meticulous, self-starter and confident.
Start wrapping it up with a recommendation to admit.
Offer to provide more information.
State the best time and place the recipient can reach you.
Proofread your reference letter and make corrections.

Explain how you know the applicant and how long you have known him/her.

In what respect is this person exceptional to others you have known with a similar background? List the applicant’s exceptional qualities and skills, especially those that are related to the applicant’s field of interest or job search. Give specific examples to back up what you have written.

Refer to the requester’s competency in a specific field and/or prior experience, organizational and communication skills, academic or other achievements, interaction with others, sound judgment, reliability, analytical ability, etc.

Omit weaknesses. If you can’t write a positive letter of reference, you should diplomatically decline when you are first approached.

State your own qualifications. Why should the reader be impressed with your reference letter?

Emphasize key points that you want the reader to take note of on the resume or application. Be sure to elaborate meaningfully; don’t simply restate what he/she has already written.

Unless it is absolutely relevant, do not refer (either in a direct or implied reference) to the applicant’s race, religion, national origin, age, disability, gender, or marital status.

Don’t be too brief, but be succinct and make every word count. Generally speaking, a letter of reference for employment should be one page; a letter of reference for school admission should be one to two pages.

List your own contact information if you are willing to receive follow-up correspondence or answer questions.

Make the ending strong without overdoing it. Undo praise can be viewed as biased or insincere.

Proofread! The letter of reference represents both you and the applicant.

Here are some additional things to keep in mind:
Appearance. Type your reference letter. Your reference letter casts a reflection on both you and the candidate. Appearance may even determine if it will be read or not. Print the letter on good quality ink-jet paper.

Specifics. Concentrate on several different aspects of the person. Be specific when you refer to his/her skills, attitude, personal attributes, contributions, performance, growth, etc. during the time period you have known the candidate.

Word usage.

Be careful with “power words”! Some words that seem harmless in every day conversation can carry both positive and negative connotations when written and presented to a prospective employer. Here are a few positive adjectives: honest, articulate, effective, sophisticated, intelligent, observant, significant, expressive, creative, efficient, cooperative, imaginative, dependable, reliable, mature, and innovative.

Avoid adjectives and adverbs that carry a mediocre connotation such as: nice, good, fair, fairly, adequate, reasonable, decent, and satisfactory.

Attributes. The National Association of Colleges and Employers compiled the following list of attributes. They can be exceptional topics to address as you describe the candidate:

ability to communicate
willingness to accept responsibility
energy level
interpersonal skills
ability to handle conflict
goal achievement
appropriate vocational skills

Intangible qualities. The ASCUS Annual listed the following intangible qualities as important when evaluating teaching candidates–a good list to consider for other vocations as well:

native intelligence
a divergent, abstract thinking style
a high level of commitment
the ability to be a “self-starter”
a high energy level
the recognition that excellence is a journey, not a destination
the potential ability to lead