Inquire with potential recommenders as soon as you know you plan to apply. If your recommenders know you very well, they should be willing to write a letter of recommendation for you. If you want to ease their burden, provide them with as many materials about your application as possible: e.g., resume, reasons why you are interested in graduate schools, etc. Approach as many potential recommenders as possible: interview them, and ask them “What can you say about me?” Select the recommenders who can best vouch for your candidacy.

Check out this blog post on more tips for your reference letters.