If you have been asked to write a letter of recommendation -- whether by a student or employee -- you must first make sure that you feel comfortable with the task. If you feel you are not familiar enough with the individual's performance or work style, let him/her down gently. A vague, lukewarm letter is invariably a disservice to the applicant. 

Even if you do feel familiar enough with the individual's work to write the letter, it is always a good idea to request extra materials, to conduct a brief interview, or both. 

EXTRA MATERIALS

If the applicant has not already done so, ask for extra materials that can give you a better sense of the individual's direction, focus, accomplishments, and future plans. Materials particularly useful for letters about academic applicants include transcripts, papers and writing samples, a copy of the Personal Statement, and/or a resume. The job applicants can submit a professional resume, a description of the coveted job, and summaries of projects completed.

THE INTERVIEW

Such an interview gives the applicant a chance to rehash his/her accomplishments, and it gives you the opportunity to get a better sense of the person's past experiences and aspirations. This, in turn, leads to a more tailored letter -- and, the more tailored the letter, the more convincing it is.
The interview should help you to clarify a number of points:

1. The Purpose of the Letter.
Ask the applicant about short-term and long-term goals, choice of school program or job, and reasons behind this choice.

2. The Applicant's Main Accomplishments.
Ask the applicant about major achievements, and how they relate to their choice of program or job.

3. The Applicant's Main Qualities.
Ask the applicant for a short list of adjectives describing his/her temperament and work style. If the letter of recommendation form specifically asks you to address the applicant's weaknesses, ask the individual to pinpoint areas of weakness that could be addressed through the program or job. 

4. Anything Else the Applicant Would Like to Bring Up.
Make sure you give the applicant the opportunity to bring up any information which might be relevant to the letter of recommendation. 

5. Practical Information.
Make sure you are clear on the deadline, how many copies/versions the applicant needs, and exactly where the finished letters must be sent.