1. Don't fail to read a faculty member's response to your request.
You've asked a faculty member to write you a letter of recommendation. Carefully interpret his or her response. Often faculty provide subtle cues that indicate how supportive a letter they will write. Not all letters of recommendation are helpful. In fact, a lukewarm letter or somewhat neutral letter will do more harm than good. Virtually all letters that a graduate admissions committee read are very positive, usually providing glowing praise for the applicant. A letter that is simply good, when compared with extraordinarily positive letters, is actually harmful to your application. Ask faculty if they can provide you with a "helpful letter of recommendation" rather than simply a letter.
2. Don't push for a positive response.
Sometimes a faculty member will decline your request for a letter of recommendation outright. Accept that. He or she is doing you a favor because the resulting letter would not help your application and instead would hurt.
3. Don't wait until the last minute to ask for a letter.
Faculty are busy with teaching, service work, and research. They advise multiple students and likely are writing many letters for other students. Give them enough notice so that they can take the time required to write a letter that will get you accepted into graduate school.
4. Don't have bad timing.
Approach a faculty member when she or he has the time to discuss it with you and consider it without time pressure. Don't ask immediately before or after class. Don't ask in a hallway. Instead, visit the professor's office hours, the times intended for interaction with students. It often is helpful to send an email requesting an appointment and explaining the purpose of the meeting.
5. Don't wait to provide supporting documentation.
Have your application materials with you when you request your letter. Or follow up within a couple of days.
6. Don't provide your documentation piecemeal.
Provide your documentation all at once. Don't offer a curriculum vitae one day, a transcript another, and so on.
7. Don't rush the professor.
A friendly reminder sent a week or two before the deadline is helpful; however, don't rush the professor. Or offer multiple reminders.
8. Don't provide messy, unorganized documents.
Anything you provide the professor must be free of errors and must be neat. These documents represent you and are an indicator of the seriousness with which you view this process as well as the quality of work you will do in grad school.
9. Don't forget submission materials.
Don't fail to include program-specific application sheets and documents, including websites to which faculty submit letters. Don't forget to include login information. Don't make faculty ask for this material. Don't let faculty sit down to write your letter and find that they don't have all of the information. Alternatively, don't let a professor try to submit your letter online and find the he or she doesn't have the login info.
10. Don't provide incomplete supporting documentation.
Don't make a professor have to ask you for basic documentation.
11. Don't forget to write a thank you note or card afterward.
Your professor took the time to write for you -- at minimum an our of his or her life -- the least you can do is thank him or her.
12. Don't forget to tell faculty about the status of your application.
We want to know, really.
Finally, remember that the general rule is that you want your letter writers to be in a good mood when they write your recommendation letter and to feel good about you and their decision to support your application to graduate school. Keep that in mind and act accordingly and you'll increase the odds of receiving an excellent letter.