Your letters of recommendation won't really separate you from other applicants. It's more about finding three people who are more important than you that are willing to take the time to write something out for you.
Best advice I got on the subject was to know going into a class (ie, start finding profs before a month before admissions 🙂), and make a point of doing well, and take the time to talk to your profeNssors.
I had an organic chem prof who offered help in a coffee place once a month, and in the semester I had his class, I was the only student who ever went. Profs appreciate someone who values their teaching enough to seek it out, and often times the subject of what you're doing after school comes up in normal conversation, and every science-professor letter I got was offered to me, not requested.
The other was from a physiological psychology professor who needed help in his lab. It was easy work, and very creepy (slicing rat brains and giving them brain lesions...), but being the only student who offered to assist put me in his head and got me another letter.
Make a point of going above and beyond with a professor, and demonstrate a good work ethic and you're golden. Most of them have written many letters before, and will often ask you if you need one if you spend enough time with them. If they've never written one, then just look online for med school letters of rec and see what the standard is, and subtly steer them that way.
I also would stress to ask your professor long before you need them, and fudge the details of the deadline. I had a hard time getting my letters on time even after giving them a month before a deadline, and even then the deadline I gave was way before I actually /needed/ them. They've got a lot of work to do, so make it easy on them and don't put yourself in a situation where you have to pester someone to get it.
A good letter won't gain you admission, the bad ones are the ones that will stand out; ie, DO NOT EVER get a letter from someone who doesn't seem to like you. Admissions committees read letters all day that say "this guy is great", so yet another letter like that doesn't stand out nearly as much as a bad one...
Reading the OP again, I realize you weren't looking for help in getting them, and just for help advising the writer as to what they should put. To reiterate, just google letters of recommendation for med school, and go that way. They're just about identical from one to the next.