The more information and assistance you give your recommender, the better the result is likely to be. The information below is necessary or helpful for recommenders; provide this information in writing if possible.
Your current contact information, so your recommender can reach you if necessary.
The name, title, organization, and address of the person to whom the letter should be addressed.
The address to which the letter should be sent. Some organizations require that letters be sent to you in sealed and signed envelopes, which you submit with the rest of your application. Others want recommenders to mail letters directly to them. Make sure you follow all directions and inform your recommender accordingly.
Information about the position/scholarship/graduate program to which you are applying.
A brief description of your goals and plans; your resume; and any information you'd like your recommender to address in the letter.
A reminder of how you and your recommender know one another; classes you took with him/her, evaluations you got in those classes, paper topics you worked on, etc. You might wish to include a copy of a successful paper or final exam, and any comments your professor gave you on your work.
A draft of your personal statement or research proposal (for grad school or scholarship applications).
Offer to supply stamps, address labels, stationery and envelopes, if appropriate. (Some recommenders may prefer to use departmental stationery for all their correspondence.)