From an email I sent to a friend of mine recently:
Let me offer some of my experience in writing reco letters. Medical schools are looking for a few keys traits such as personality, reasoning skills, ability to adapt to new situations, empathy, warmth, genuineness, research potential, as well as research history. I don't expect you to list the finer details of the work I've produced in a lab. Instead I think you should try to focus on what characteristics you like to see in a doctor and how I would meet them. For example, do you want a doctor who can read your mind and figure out your problems without your ever having to utter a word? If yes, recall the times that I've talked to you and how I could name your feelings without much input. Do you want a physician who can explain how a disease, drug, or procedure works in a manner that is easy to understand yet does not compromise the scientific integrity? Then recall all the times I would talk about physics.
Admissions committees also like to read letters with lots of action. Nothing puts a reader to sleep quicker than a list of personality traits without specific stories given as evidence. You could say that I'm reliable and have a few people believe you. Or, you could talk about how I edited a couple of PDF's so you could apply to law school and later for high honors. Mention that I brought you to the physics building and introduced you to several majors to drum up votes for your senate campaign. Talk about my sole stance against (person) and giving you the nomination for president of the (our club). Now, everyone will believe you. I think that if you find four or so similar combinations of characteristics + stories you'll be fine. Add a "Ryan Through the Ages" segment between each paragraph and you've stepped upon a gold mine.
