Believe it or not, very few students take the time to thank their recommenders. Take a few minutes out of your day to handwrite a thank you note to each of your recommenders. Why? Writing a good letter takes time and energy. The writer must consider the entry requirements for each graduate program and consider your background and experiences to determine how to write the most effective letter. He or she is not required to write a letter on your behalf and instead is going out of the way to help you. Reciprocate by sending a thank you note that expresses your appreciation.
Given that few students send such notes, yours will stand out. Why do you care? Because the recommendation letter for entry to graduate school is just the first of many letters that you will need over the years. You'll need letters when you apply to fellowships, some scholarships, and eventually when you apply for jobs after graduation, as well as awards later in your career. Not to mention, you might decide to apply to more schools later on, or even next year. Also note that sometimes admissions committees call faculty who have written letters of recommendation. Make sure that your letter writers continue to view you in a positive light.