So you would like me to write you a reference...
If you would like me to write you a letter please first look over this overview of the process (from a faculty member at the University of Texas-Dallas).
Another good source is this article in Science:
Once you are sure you fulfill these criteria then you should ask me (in person) to write you a letter. If I agree I would like a list of your programs, areas of study, addresses and due dates. I also would like a soft copy (email or word processing document is fine) of the addresses of each of your schools. That way I can paste this information into my reference letter (and keep programs straight).
You should give me all of your materials at least a month before the first due date. You should also initiate the application process for all of your schools at the same time. That way if/when electronic references are required I can get them done all at once. Given I write letters for a number of students each year it makes it easier for me to systematically finish all references for each student.
My agreement to write a letter is predicated on you waiving your right to see the letter. It is my opinion that admissions committees view such letters as more honest and open. It is your choice in any case but I choose not to write a letter if you do not make the reference letter confidential.