I have written a number of LORs for the research assistants I supervise, and here is some advice from the perspective of the writer.
A packet put together that contains a cover letter, CV, bulleted list of accomplishments, and any forms needed (eg waiver) serves several purposes. First, it shows organization, second it insures that points that you feel the writer should include won't be forgotten, it also insures that the waivers won't be lost. Also include a prestamped and addressed envelope and a few sheets of letterhead if you have access to it.
Basically, remember that the easier you make it for your letter writer, the faster you will get a great letter. Just my $.02.