Provide your professors with a list of graduate schools where you will be applying. Give them a comprehensive list, including the names of the programs, the names of the heads of the programs, the addresses and the deadlines for the letters.
Explain each program in detail, preferably in writing, for your professors' reference as they prepare their letters. This is especially important if you are applying to programs in more than one academic area, or if the degree program is not your professor's specialty. Let your professors know that for similar programs, you do not mind if they use a similar letter and readdress it for each school.
Offer to meet with your professors to discuss each program and your career goals. Bring your resume when you meet so you can cover the highlights of your academic experiences so far.
Provide the professors with pre-addressed, stamped envelopes. This will save them a lot of time if you are applying to more than one school.