I gave each of my letter writers a folder containing a short cover letter, copy of my transcript, curriculum vitae, the AADSAS form, and a pre-addressed envelope (and proper forms) to my school's LOR file service. I also made sure to meet with them, so they could ask any questions or get to know me a little better/refresh their memory. I think they all appreciated having the information in front of them, even if they decided not to reference all of it. I was advised by the career center to ask the profs point-blank whether they felt comfortable writing a strong letter for me. I couldn't get up the nerve to do it, but I think it's a good idea -- I think professors will generally be honest, and should WANT to write you a great letter... if they can't see through a couple bad grades or minor flaws, it's important to find someone else!