Your first step is to talk to the professor about what the letter is for. Tell us when the deadline is and ask if we would write a letter of recommendation for you.

Next, give us the following. These items are not mandatory, but they help us to write you a stronger letter.

1. List of courses with that professor, when you took them and any particular projects or presentations you did in that course.
2. Transcript(s) OR list of courses and grades relevant to the program/job to which you are applying.
3. Co-curricular and extra-curricular activities relevant to your application.
4. Foreign and/or inter-cultural experiences (e.g. OCS, overseas or domestic volunteering and homestays).
5. Your resume if you have one. 
6. A draft of your application essay, if you have one, so we can better coordinate our letter to your expressed purposes.
7. A short description of what you think should be included in the letter. We won't feel obligated to mention everything you tell us, but we can't include it if we don't know about it.
8. Any materials necessary to submit the recommendation letters.  If you are applying to a graduate school that uses paper-based letters, be sure to provide us with the appropriate forms.  For a job application, you might need to supply an email address or a postal mail address to send the letter. If we need to send anything by postal mail, please supply addressed and stamped envelopes.
9. A summary page listing all the places you want us to send a letter, what we need to do, and what the deadlines are. Sorting these in order of deadline makes it easy for us to get the earliest ones out first.

Finally, here's your end of the bargain: after you've heard back, tell us how it turns out. This helps us learn how to advise future students.

Thanks, and good luck!