David A. Sibley, author and illustrator of the premier field guide to birds in North America has been on a roll recently, writing great identification related posts on his blog. Most of them have dealt with subspecies identification so if that is something you are interested in, you should probably check it out.
Some of his recent posts include:
Distribution of Greater White-fronted Goose subspecies
Distinguishing the subspecies of Purple Finch
Can subspecies of Mountain Chickadee be identified in the field?
He also has several posts regarding identification by behavior: