when asking for a letter of rec, you pick someone who knows you well and will give you a good letter ... it helps if they are somewhat nationally known for something (so there is name recognition) ...
as far as format, i don't know of any general format ... most people have there own way of writing LOR's ...
as far is what we look for in an LOR,
1. first you need to sign the "i sent this without looking at it" form ... if you don't, then it brings some questions .... i have only seen several people who didn't sign it ... and no interview was offered
2. i look for key words, things that make a point to separate you from other applicants they might have written LOR's for ... things like "the best medical student in the past 5 years." or in the top 5% of medical students rotating here. the line, we are highly recruiting him/her here ... for me, doesn't mean as much because i have seen several people with that same statement who were not even interviewed at that program ...
3. negative statements, this is the tolling bell ... if you had a bad rotation with someone and you think they may write you as letter, do not ask for a letter ... even if they have a big name ... negative statements will kill your application .... i saw one recommendation that basiclly said, the applicant has strong will and we tried to convince him/her to seek another specialty because orthopaedics was extremely competitive, but he/she would not be swayed. ... that's bad
hope this is helpful