I gave my letter writers:
1) A personal statement identifying the field and why I chose it, what makes me a good candidate and what my goals are in that field. Mine was 2.5 pages, too long by most accounts, but very helpful to those who wanted to write a comprehensive letter consistent with my goals.
2) A 2 pg CV with academic, volunteer, work, biographical data, research, and grades for 3rd year broken down by test/clinical/overall, and step scores. Specific but brief. No HS/college stuff unless it was pertinent to med school work.
3) ERAS cover sheet waving my right to see the letter and providing the name of the person to recieve it and fax number.
4) Target day agreed upon, with friendly follow up email reminders.
5) Emails after interview season and match to let them know what happened, and thank you notes.
There may be other things that are useful, but I'm sure your letter writer may ask. Keep in mind their level of experience writing, if this is young faculty they may have not written many, and ask yourself if this person is particularly sympathetic and eloquent, both features that lead to well written, supportive letters.