I would schedule an hour-long meeting with the recommender after you give them the
AAMC LOR brochure*, your PS draft, a resume (which you'll convert to your experiences section: please break down by type of experience in a nice indexable way: healthcare, community service, extracurriculars, etc.). Find out what your reference wants to help with the letter. If you have a prehealth advisor (especially if institutional letters are available) or career advisor on your campus, they may also give you advice; premeds aren't the only people who have to ask for LOR's.
Conclude your conversation by flat-out asking:
Can you submit for me a VERY STRONG letter of recommendation by [early May/June/July/whatever] ?
Also don't forget to plan for a nice thank-you note and gift.
* Bonus points if you can also summarize/highlights of areas where your activities fulfill the qualities/competencies specified in the brochure in your resume. Make things easier and show you've actually READ the brochure. You'd be amazed that people don't do that extra step and expect the recommender to figure it out. (Not that the letter comes out badly, but you may lose a critical opportunity to get a really strong letter.)