Being asked to write a letter of recommendation can be both flattering and annoying. It does take time and energy, but if the requester is worthy and you're able, it really is the right thing to do.
If you're asked and accepted, ask the person what skills or qualities they'd prefer you touch on in a reference letter. You could also ask the person for materials and background that will make the letter-writing task easier. For example, school transcripts, a resume, a list of strengths and goals, and so on.
Find out how long the letter should be (one page is typical), if there are any formatting requirements and when it's due. If you commit to writing a letter, follow through. The requester is counting on you, and a late or weak-effort letter could torpedo their chances.
In the letter, be sure to detail how you know the person (he was your intern, she was your teaching assistant, you worked on XYZ project, you're his mother, etc.) Include your credentials-and don't be modest. Your stature is an asset to the person for whom you're vouching. To be of help, you need to show that you are credible and your perspective is relevant. Mention how long you've known the person, and give specific, concrete examples and anecdotes that illustrate his or her talents, experience, passion and/or character. Be aware that in an employment or academic letter, if you put all your focus on just one or two attributes or skills, you run the risk of making the candidate look one-dimensional; it's better to be well-rounded.
Similarly, if you don't have time to write a reference letter, or to write one that will do the candidate justice, it's best to decline. It would do more harm than good to present a sloppy letter written in haste. If you feel uncomfortable vouching for the person at all, due to shortcomings on his or her part, just decline. It's better to let the person know outright that you can't give them a good recommendation than to write one that's lukewarm or even negative. Perhaps you just feel the person's skill set is not a good fit for this particular job.
If you're asked to write a reference letter you feel you don't know very well, that's not necessarily a deal-breaking, depending on the circumstances. Some college professors, for example, proactively write letters of recommendation for top students immediately after they take their class, so their attributes are fresh in their mind.
Keep in mind that not every reference letter must be a fabulous "glowing letter of recommendation." You can recommend someone without sounding over-the-top or lukewarm. You don't have to say you're "very pleased" or "honored" to recommend this person; you can just be plain "pleased." You can also "highly recommend," "recommend without reservation," or just plain "recommend."
Finally, make sure you're authorized to write letters of recommendation. Some companies forbid them for liability reasons — a subsequent employer could sue an earlier one for failing to disclose something about a candidate, or for misrepresenting him or her in some way.
Proofread your letter before sending it or giving it to the person. If mistakes are found, correct them. Consider including contact information in case whoever receives the letter has follow-up questions for you. Finally, use good-quality paper.
Index of letter of recommendation templates