Do you screen (talk to) several (ex. 5 or more) potential referees, then choose the 2 or 3 from a much longer list?
I think having 4 - 5 letters/letter writers is a good idea. I don't know what the LOR requirements are for the schools you are applying for, so you might want to check them out. Some schools want, for example, 3 academic letters, at least 2 of which should be from science faculty. Other schools want at least one non-academic letter. Some schools want X amount of letters by certain people, but allow you to send in extra letters. I would make sure you have at least three academic letters (maybe four), at least two of which should be science. For additional letters, you can seek one or two non-academic letters if you like.
How long before submitting applications do you check out potential referees? A couple of weeks, several months, â€¦
At least a couple of months. There are some online reference services that are wonderful that allow your letter writer to submit ONE copy of their letter to the service and then you can request that copies be sent to your schools as the schools request them. This makes it nice for your writer, because they only have to write/send a letter once, and nice for you because once they have submitted that letter, YOU have control over when it gets sent to admissions committees (rather than having to contact a writer every time you have a school request letters).
Does one bluntly ask a potential referee: â€œCan I expect a positive letter of reference from you?â€
Most people will indeed suggest that you ask your potential writer "Would you be able to write a GOOD letter of reference for me?" If they seem uncomfortable or hesitate, then you have your answer and even if they eventually say yes, it might be in your best interest to drop it with that person.
Do you consider former business associates, former supervisors among your candidates?
One or two letters from someone who knows you well is probably sufficient, but again, schools usually prefer academic letters, so make sure you have those covered first. If you can't get the academic letters (say because you've been out of school for a few years), then you can usually request that the school consider professional/personal letters. I had one personal reference, but only ended up submitting it to one school because the other schools were very specific about wanting certain types of letters.
How far back do your last dealings with your referees go? Less than 1 year, more than 5 years? Somewhere in between? How long ago is too long?
This is just my personal opinion, but I would say that you shouldn't go too far back (say more than a year or so) unless you think that person will write you a truly outstanding letter.
Is a Math professor you had for one full year, better than a chemistry professor you had for only one semester? In other words, does duration of â€˜contactâ€™ make a difference?
Maybe, maybe not. Again, check into what LOR requirements your schools have. If they ask for three academic letters, at least two of which should be science, then you can use both the chemistry AND math professor. I would tend to say that the quality of the contact is more important than the length of the contact. How well do they know you on a personal level?
My volunteering supervisor (local ER triage area) seems to have an 'attitude' with respect to med-students - who he is partly responsible for evaluating. I'm counting on him as a reference; how do I go about sizing him and making sure he will refer positively?
If you don't feel that this person can/will give you a good reference, then don't use them. You may not even need to send in a letter from a non-academic person, and if you do, a glowing letter from a supervisor is going to be better than a mediocre letter from your volunteer supervisor.
Hope this helps.