Asking for a Recommendation Letter
Many students ask me to write them a recommendation letter, whether for a job or internship application, or for graduate studies. I usually decline, unless I have worked with the student in a project before. Here's why.
Concrete Examples
To write a good letter, I need to be able to give concrete examples. General comments, such as, "This student is hardworking and smart", are weak and useless. Everybody applying for a job or for graduate school is hardworking and smart! That goes without saying. For the applicant to stand out, I need to be able to say something like this:
"Faced with the problem of automatically locating facial landmarks in an image, Joe tried three different methods. First, he downloaded, compiled and ran an open source landmark detector, which unfortunately gave poor results. Joe then tried to re-code a key part of the algorithm, the template matching step, but this did not significantly improve its precision. Finally, Joe decided to code from scratch: he read up an OpenCV tutorial to learn about pattern matching using illumination invariant features, and then wrote his own landmark locator. This proved successful, and Joe was able to continue with the rest of the project.
I was impressed with Joe's perseverance in overcoming this problem, his resourcefulness to scour the web for solutions, and his willingness to learn new skills to get the job done. Many students would have given up, but Joe took on the challenge without complaining. Joe may have lost sleep for a few nights trying to solve the problem, but he's won my admiration for eternity."
As you can see, to be able to cite such specific examples, I need to have worked with you closely over a semester or longer. If my only interaction with you is just through teaching a class that you attended, then I really can't say anything beyond what is already obvious from your academic transcript. And your transcript is at least more complete and concise, and hence will pack more punch, than what I can write. Doing a class project doesn't help either, because such projects are usually done in a group, and does not allow me to supervise you closely.
So having taken my class, no matter how well you did in it, is not a good reason for me to write for you.
Weak Letters Hurt
Let's suppose I were to write a general and emasculated letter for you anyway, upon your insistence. Such a letter will actually hurt your application. It tells the reader that since this is the best you can provide, you must have been a bookworm: someone who is always buried in your studies, with only superficial interaction with your professors, most likely someone without extra-curricula activities. You will therefore come across as parochial, lacking in initiative and passion, and perhaps weak in EQ. These are not qualities that will endear you to your future employer, or to professors of your graduate school.
I have been on the receiving end of such letters, so I know how effective they are. From time to time I have to screen applicants for our School's graduate program, or for some special course or award, and I have to read recommendation letters furnished by these applicants. Unfortunately, letters that spout only niceties and generalities are plentiful, and they cut no ice with me. When a letter starts off with, "I have known this student for three months; she took a course I taught last semester", and ends with, "Therefore, I highly recommend this student to you", its credibility sinks so low as to be laughable. I usually reject such applicants, because they simply do not stand out.
Some Advice
So what can you do? My advice is for you to ask letters from people who have worked closely with you, and who can cite specific incidents to substantiate your good qualities. Clearly, this requires that (a) you do something or exhibit some strength of character to impress your referee, and (b) you cultivate a relationship with him/her ahead of time. As a Chinese proverb goes, "As the long road reveals a horse's strength, so does the passage of time reveal a person's heart". Give time for your referee to get to know you. Suddenly accosting the professor, whom you have been cold-shouldering since you last took her class two years ago, in order to request for a letter the day before it is due is most assuredly not going to produce that glowing testimonial which you hoped for.
On the bright side, your referee need not be a professor; instead, he could be your internship supervisor, or the coach of a sport in which you excel, or even a religious leader who can vouch for your spirit of compassion and volunteerism. The key is for him to be specific, not general, when he writes.
Also, don't assume your referee knows how to write a good letter. It doesn't hurt to help your referee to be more specific, although you should be tactful in doing so. Prof. Shriram Krishnamurthi provides an excellent, albeit long, advice to letter writers here. Looking at various samples of letters will no doubt also be useful.
Remember that your letter should help the reader assess you holistically, not just in your academics. Your future professor or boss wants to know how well you work under pressure, whether you are a team player, have you led a team before, how you handle setbacks and failures, etc., in addition to your technical skills of debugging code, understanding math, and solving problems. So be sure to provide ammunition for your referees to shoot on your behalf. Give him/her your updated CV. Speak about both your academic and non-academic achievements: Have you volunteered to serve the poor in your community? Or climbed Mount Everest? These are just as significant as getting on the Dean's List. Better still, tell your referee if you have failed at a task, and how you rebounded and subsequently overcame the failure. Tough situations in the past demonstrate that you are authentic, and that you can probably handle tough situations in the future.
Conclusion
Now that you know more about what makes a good recommendation letter, are you sure I am the right person to write for you?