1) Ask professors who know you well and have a high opinion of your work. Since each professor writes a number of letters each year, we cannot truthfully say that every student is the best we’ve ever taught. If a professor agrees to write for you, he or she will almost certainly write a positive letter; but you should expect that letter to reflect your good qualities in an honest way. If you suspect, from your interactions in workshop and conferences and from your professor’s comments on your written work, that your professor may, for example, have found you talented but lacking in motivation or unreceptive to criticism, your letter may reflect this fact.
2) In my view, you are better off requesting a letter from any teacher who knows you well than from someone famous with whom you have only a passing acquaintance. At least one letter should be from a professor in the field for which you’re applying for graduate study, but an insightful, positive letter from someone in another discipline can also be quite valuable.
3) It’s often best to request a reference from a professor with whom you’re not currently studying. Being able to evaluate your work over time, including your dedication to final work/revisions, is an important part of writing a recommendation; and I, for one, don’t always feel able to recommend a student’s work after only knowing him for half a semester. If you’re currently studying with a professor with whom you’ve studied before, this rule would not apply. There are also circumstances in which a letter from a current professor can be just fine. (I’m thinking specifically of applications for the Writing Concentration at Yale—an internal application process.) If, for example, you are currently doing strong work in Advanced Fiction, that professor might be a more logical choice to recommend you than the professor with whom you were just finding your voice in Intro.
4) Give professors plenty of time to write your letters. A student once asked me to write a letter a day before its deadline—which was both impossible, given the demands on my time, and disrespectful of those demands. Professors like to know as far in advance as possible how many letters they’ll need to write during letter-writing season. Telling a professor in the summer that you plan to apply to grad school that fall would be ideal. Four to six weeks to write the letter would be an absolute minimum. Remember that, if you want your letter to be good, your professor will have to craft if carefully.
5) Try to be understanding if a professor declines to write for you. He or she may already have undertaken to write a large number in a given semester, or may honestly believe that another professor could speak more eloquently about your abilities.
6) Acknowledge the time commitment each letter will take, even if a professor can upload it via a university website. (A good rule of thumb would be to assume that your professor will spend one hour uploading your letter to each school you choose—if the university’s system is functioning correctly at that time. Understand that this is not because your professors are older than you and not digital natives, but because university application websites are byzantine. See below for elaboration.) If a school asks you to provide a professor’s e-mail address, you might infer that the school intends to contact her casually if you make the first cut. But the moment you provide that address, the institution sends her an automated note requesting a formal and fully articulated letter of reference.
7) If physical letters are required, provide your professor with properly addressed, stamped envelopes. Providing a list of all schools you are applying to, along with application deadlines, would also be helpful.
8) As long as your request is polite, it’s fine to ask a professor for a letter in an e-mail. I’ve recently had some well-mannered former students arrange meetings with me for this purpose; and while I was delighted to see them, I understood from the get-go that they wanted to request letters, and would have considered it perfectly polite of them to ask up front.
9) If your professor is on family, medical, or parental leave, there’s a good chance that now is not a good time to ask for a letter. Of course you can always try, but it’s good to remember that someone who’s out on one of these kinds of leave is grappling with major life issues and may simply not be able to help you (or even respond quickly to your request for help) at this time. A professor out on sabbatical is getting some hard-won time to work on her own research and writing, and may also prefer to keep letter writing to a minimum; that depends on the individual.
Please note that, as of the posting of this advice on my website, I am no longer willing to write letters of reference that require engaging with universities’ online reference systems. I will gladly file your confidential letter of reference with Interfolio (or another dossier service), which can send it either by mail or electronically to any institution to which you apply. (As of this writing, Interfolio can deliver a letter directly to most universities’ online systems.) Your undergraduate residential college or house can in many cases provide a similar service.