Compile a list of people you think are qualified to write you a strong letter of recommendation. This list should include past supervisors and former professors who know how you work and can paint a positive picture of your scholastic aptitude.
Contact the people on your list and set up a meeting to discuss your goals and reasons for attending graduate school, and to request a letter of recommendation. If someone refuses or shows reluctance, thank them and move down your list to the next person.
Provide those who agree to write a letter with your contact information, what you would like emphasized in the letter, the list of schools you are applying to, recommendation deadlines and any other information you feel relevant. Also provide them with a copy of your Statement of Purpose, your resume and stamped envelopes addressed to the schools to which you are applying. Give your references plenty of time before the deadline to go over your information and write an informed letter of recommendation.