Familiarize yourself with the position. Get a copy of the job description, if you can, and talk to the person you will recommend. If you know the intended recipient of the letter, ask them about the position as well. The more you know about the position, the better you will be able to tailor your letter to suit the needs of both parties.
Why should I use the Letter Service at the Career Center? If you are applying to more than three schools, or you think you want your letters held for other opportunities (like law school) in the future, you should plan to use the Letter Service. Your letter writers will write one letter and fill out one form, rather than filling out a different form for every application.