Mark-
No matter what you do to remove the background, you still need to get the exposure right. As I said before, with white on white, you need to either use the manual exposure more or exposure compensation in order make your image really white. Your light meter wants to average all values to medium gray. That works fine for most scenes as they have a lot of tonal values. But for white on white (or black on black) it will make everything gray. You need to overexpose by a stop or more to get the image to look right. If you do that and then use the eyedropper tool on the background, you'll have a lot less work when you try to select the shirt.