Since I wrote this short joking post about faux pas in a friend's recommendation letter, I've been getting people asking me how to write a good recommendation letter.
For posterity and the Internet, here you go:
Remember, your friend / colleague / student is submitting this for something that is very important to them. Probably the most important part is making sure that it reads like something you'd submit to a scholarly journal, not something you'd comment on their myspace page with.
Talk about them as you know them: write about the qualities they've shown through the experiences you've had with them. Don't fluff, bullshit, or otherwise go further than your experiences can warrant.
Remember, they're trying to get something from someone with this. That job, college admission, or grad school means a lot to your friend, and you have the ability to directly affect their chances. On the other hand, don't advocate so much that you're written off as a cheerleader.
If prompts are provided, make sure you nail every one of them. If they aren't, think about what you'd want in someone working or studying with you, and nail those points. Particularly, make sure you talk about communication skills, work ethic, and creativity, since those are pretty universally applicable.
Make sure you discuss ways in which they're not perfect. Talk about how they worked on their problems, how they changed for the better, and how they overcame obstacles.
Before you send that make-or-break document off, have other people read it and give you feedback. A fresh set of eyes can catch possibly credibility-damaging errors before you have to apologize for them.
In a pinch, drop me (Justin George) a note and I'll do a brain-dead check on it for you.
Stylistically, there are a couple things you want to make sure you do:
Write in short, complete paragraphs, and make sure each one has a topic. Limit yourself to a few sentences for each, and keep it punchy.
Take the space you need, but don't be verbose. Imagine yourself in the shoes of the reviewer: you don't want to waste time, neither do they. I usually suggest that people aim for a page, two or three if you've known them for a long time or in multiple roles.
Sign it with real pen, on real paper, and mail it to them. People are silly creatures, and an authentic signature on good stationery will make it clear that you mean it.
You do have good stationery, don't you? Everyone should have a few sheets of quality personal stationery for just such an occasion. Rag paper with a heavy feel shows you care enough to spend money on the people you're writing to, as well as the person you're writing for.
These are, generally, the same rules you should follow for all good writing. In fact, you'd do well to follow them for all official correspondence, including admissions letters and particularly thank-you letters.
If you're the one getting a recommendation, remember that it takes a lot of time and effort to write a really stellar one, so make sure you write them two letters: One to ask for a recommendation letter (Even if you're asking for a recommendation letter in person, it's a nice touch to follow up) and a second as a thank-you letter (both on real paper, following the rules above) and mean it. It will make you memorable in the future, and that can mean jobs, referrals, and other benefits, as well as a lasting friendship.