If you want a letter of recommendation, I would appreciate the following:

1. Please ask me if I feel comfortable writing a letter for you.  I will let you know if I feel I can write you a positive letter. 
2. Ask at least three weeks before you need the letter(s).
3. If it is an option, waive your right to read the letter(s).
4. Provide me with information that would be helpful for your letter.  You might give me a copy of your cover letter and CV.  In addition, you could note specifically what qualities you think it would be most helpful for me to emphasize. 
5. Organize all the information I need into one physical or electronic document.  For example, if you want me to write letters for three different graduate programs, please send me one email that has links for all the electronic letters, and includes as attachments all the things I might need to write letters for you. If you need physical letters, please put a folder in my box with self-addressed, stamped envelopes. 
6. I will email you when I have submitted/sent letter(s).  If you do not hear from me by one week before the due date, email me a reminder.