Several weeks ago, a student I've taught in one class came in and asked if I'd write him a recommendation letter for grad school if one of his others from his summer internship manager didn't come through. I said I would. Well, late last week (the 22nd, I think), he came back and asked if I'd write the letter, and then proceeded to tell me that the deadline for submission of his materials was February 1st. It's a trite phrase, but lack of planning on your part does not constitute an emergency on someone else's part.
If you're going to have someone write you a recommendation letter for graduate school, here are some things that I would consider basic etiquette for you to do. Even if they know you pretty well, these are some basic courtesies that will probably make them think that much more highly of your level of professionalism.
1) Don't wait until the last minute. Give them plenty of time.
2) Provide them with a copy of your resume so they can see what you've done. They may not know everything you've done. With my student, I know he did one internship last summer, but other than that, I've no idea. I'm not even sure if he is a double major or works a job in addition to going to school and being an athlete or what exactly his career goals are...no clue.
3) I almost always give the student a copy of the letter, but if there is a form from the university that asks if the student can see it, I appreciate the fact when a student checks "no" ahead of time. This student filled out the form in front of me, checked "yes" and said, "I'm sure I'll see it anyway." I hadn't told him I'd give him a copy. When I write letters I put them in an envelope and sign the flap so that the school knows the student didn't forge or alter the letter. It's pretty common practice. This schools instructions even stated to do that. He for sure wasn't going to see that copy. It was pretty ballsy, I thought, to assume that he would get a copy. He didn't ask for one. If you don't specifically ask for one, don't automatically expect to get one.
4) I would have appreciated if he had taken even 5 minutes to talk to me a little bit. I would have been interested to hear (and would have asked) why he wanted to get into that program, what he thinks he might want to do in the future, what other experience he had is sports, etc. He just rushed in, asked if I'd write the letter, and then dashed out. He returned the next day to give me the form that went with the letter.
I was able to write a positive letter, but it wasn't nearly as detailed and specific as it could have been.
I'd be interested to know what else other people may ask/expect/require from students who ask for recommendation letters. I know some people make the student write the first draft and then edit from there. That's always a possibility, too.