Here's my schtick:

1. You write up a listing of what makes you great, focusing on things that I've had an opportunity to observe in you, and things that you hope I'll address in my letter. This list should include things like personal traits, specific courses you've taken with me and how you did in that situation, independent study projects you've done with me (or summer research) and how you did in that situation, and perhaps non-curricular activities as well. Among the things you might want to address are how you work with other people, how you work independently, and how you deal with adverse situations (such as difficult project partners).

   This is obvious, but be sure to highlight your individual strengths. For example, are you a great thinker, a programming whiz, meticulous with your work, especially creative, an inspiration to other students?

   It's good to throw in a "weak spot" or two as well (makes the letter more believable and you more human), and especially if there's a weak spot and evidence that you can cope with it. This might be something like that you're too picky and that makes you have a hard time getting work in on time, that you're shy and don't always speak up in class discussions, or whatever.

   This "why I'm great" list is admittedly awkward to write, so let me explain the point of it. On the one hand, you have specific reasons for asking ME to write a letter for you, and you've probably been thinking about that (or you should have been :-) and so it's all much fresher in your mind than it is in mine. Making the list helps make sure I don't forget anything. The second thing is that it gives you a good sense of what's going to be in your letter, because if there's something I disagree about, I'll let you know.

2. You might also want to make it clear what sort of opportunity this is that you're pursuing and provide additional information about yourself that's relevant to this specific opportunity. (This is especially relevant if you've studied CS with me, but are pursuing a non-CS-related possibility.)

3. Send me a listing of places and deadlines. You can also do this incrementally, as more opportunities arise.

4. Send me regular reminders about writing the letter. When I've actually done it, I'll send you email to let you know; until then, assume I haven't. And also know that I Do Not Mind the Reminders! I just have too many things going on, and this ensures that I don't forget.

5. Perhaps this is obvious, but it's the first letter that takes the most time, after that adapting it for specific places is cake. So don't worry about asking for multiple letters.

6. No, I'm not going to put a letter on file with career services. Just ask me when you need another letter.