i agree with the other posters here - the letter writer has to know you, beyond your work together. i can't tell you the hundreds of crappy recommendation letters I have read, from the typical premeds asking the organic chem professor to write a letter, that basically says "So and so was in my class. s/he got an A. s/he was #2 out of 215 students. s/he will be a good physician." To think how much time that premed spent running around, worrying, trying to get a letter...only to get that sh$# thrown to a committee...it just ain't right...
When I ask for recommendations, I ask the letter writer, "Can you write me a GOOD letter of recommendation? (though I can't define what GOOD is, they often can because they have gone through hoops to get to where they are now, and know what's involved)" If they say no, and I REALLY want them to write a letter, I ask them what they are concerned about -- i.e. do they need to meet with me a few times to get to know me, do they want a personal statement, a resume/CV, something...or do they just maybe not write letters.. If they say no and I really don't care either way, i find somebody else.