My neighbor gave me a poinsettia that has been left out in the yard all summer. It is bushy and green and doing well but how do I get it to flower now?

Day length is the key thing (needs short days to initiate color). There is some information that you might find useful from Penn State at:

http://hortweb.cas.psu.edu/courses/hort450/Poinsettia.html

Note the comment near the bottom of the flowering section to the effect that it doesn't take much light to prevent flowering. From what I vaguely remember from a long-ago course, that means the plants have to be kept in total darkness for 13-15 hours a day, so just putting them in a dark room is not enough if someone might accidentally turn on the lights every now and then! I gather the best thing to do is put them in a dark cupboard for the period required each day and make SURE no one opens the door! Although, this article also qualifies that by saying maybe it takes up to 4 hours of light interence to prevent blooming, so maybe it's not as strict a thing as I recall one of my professors saying in the 1970s... In any case, it'd be fun to try bringing it back and, if it doesn't work, it's always easy and not too expensive to buy another one for Christmas color....