You won't find a better or more complete answer to this question than the one given here, by Professor David E. Keyes. Here is the key paragraph:
Likely reference writers (for instance, well-known professors of core courses) are sought out by many well-qualified candidates. To ensure that such a writer is well primed to execute your reference efficiently, you should create a self-contained packet containing all the information the author will need to dispatch the reference in one sitting: (1) contact information for the recipient of the letter, (2) a description of the position and application closing date, (3) your own application essays and cover letter, (4) a resume, (5) relevant transcripts and scores, and (6) an explanation of the niche of the writer! It is very useful, as a reference writer, to receive a reminder along the following lines: "Professor Keyes, your letter will be the one that comments most authoritatively about my analytical ability, my promptness in completing projects, and my reasons for wanting to pursue X next fall. Remember that you gave me an A– in partial differential equations two years ago and it was your suggestion that led to my summer at Los Alamos with Y." You should provide this packet in both hard and soft copy. Writers of lots of references maintain files that may be hard or soft, or both, and you should make it easy for those writers to locate your files quickly for subsequent updating and future requests.
Some faculty write many dozens of letters of reference during peak months, and they may even ask candidates for sample text to be incorporated into letters, to ensure that they capture their niches. You should not be flustered at such a request, and should not be modest in complying. You should be aware, though, that your words will not pass directly into the delivered product; they will be used simply to get the author's juices flowing following the formulaic paragraphs of the letter.