Don't hesitate to ask! It is a lot of work to write a letter of recommendation, but it's rewarding work: I truly enjoy the chance to reflect on my students' growth and recall their accomplishments. That said, here are some things you can do to make the process easier for me--and for you.
1) Please make sure I'm the right person to write the letter. Think about the kind of letter you'll need. If you're applying for an internship or a job, you'll need letters describing your character, your temperament, your work habits; if you're applying to medical school or law school, you'll need letters describing your writing; if you're applying for graduate school or fellowships, you'll need letters saying that you were, at the very least, the best student in a given class. I want to write strong letters for all my students, but I'm not always in a position to testify to those particular things--it's hard for me to talk about your work habits, for instance, if I know you from a lecture course.
2) Please give me the time I need to write the letter. Letters for undergraduates take two weeks; letters for graduate students take at least a month, and if you're a graduate student applying for academic jobs, please start consulting with me a few months in advance.
3) Please give me the information I need to write the letter. I'll need information about you (copies of papers you wrote for me, a resume or cv) and information about whatever you're applying for (descriptions of the job or program). I'd also like to know how you're presenting yourself, so please send me a draft of your cover letter/ application/ statement of purpose--that way, I can coordinate my description of you with your description of yourself.
4) Please do as much of the secretarial work as you can. Most letter of recommendation forms, whether on paper or on-line, require the recommender to answer many little questions. Answering those questions takes time--a lot of time, actually, since I write for many students and each student ends up applying for many things. I'd prefer to spend that time perfecting my letters, so I'd appreciate it if you filled in as many of the blanks as you can. (I'll answer the questions about how long I've known you and in what capacity, as well as all the ranking quesitons, of course.) For your reference, my title is "Professor," my department is "English," my mailing address is "English Department/ Barker Center/ 12 Quincy Street/ Cambridge, MA 02138," my phone number is "617 496 2235," and my email address is "firstname.lastname@example.org." If they ask for still more information, feel free to make things up.
5) Please waive your right to read the letter. Letters aren't taken seriously if they're not confidential. And you can trust that I won't agree to write a letter for you unless I can write a strong one.
6) Please send me a reminder email two to three days before each letter is due. This is crucial. I'd hate to miss a deadline, but it could happen without a reminder.
7) And do let me know how it all turns out. I'll be curious to hear!