The letter of recommendation is an important component in the process of applying to graduate schools. So it is important to take into consideration, possible individuals that you would like to write reference letters for you. Do not wait until the last minute because you need to provide these individuals with enough time to write the letter.
Below is a great handout from Seattle Pacific University entitled:
RECOMMENDATION LETTER ETIQUETTE: Guidelines for Requesting a Faculty Reference http://www.spu.edu/depts/eng/documents/RECOMMENDATIONLETTEREtiquette.pdf