Schools typically require 2-4 letters of recommendation from current or former professors. In choosing your writers, remember that your graduate school is looking to assess your potential as a student and a scholar, so letters from faculty are stronger than letters from past employers or supervisors. Letters are more valuable to the evaluator if they are written by people who know you well and can evaluate your work. (Do not focus on people with important titles- solicit writers who know your work and can provide examples to illustrate your strengths.)
Provide your letter writers with a resume, list of activities, your graduate school essay and other pertinent information that can help them write a strong letter. Make an appointment with your writers to discuss your goals and reasons for applying to schools.
Give your writers a deadline and continue to check-in with them to see if they have completed the letter. To allow ample time for a response, ask your writers early in the fall semester or beforehand if he/she will be on sabbatical.