have an interview with the writer and give him/her some data about yourself. give them also a sheet summarizing your accomplishments and outside activities. anyone who agrees to write a letter is likely to want to help you, but they need as much help as you can give them. as a letter writer myself, the one thing i am likely not to need help with, is the little story about some memorable aspect of the person's interaction with me. i always try to think of something like the time the person helped me by correcting a mistake i made in class, or gave a homework solution better than mine,... the writer already has your grades, your class performance, your office interviews.