Ol' Myrt here spent the better part of the morning in Second Life talking with a researcher from Montana who faces challenges of identifying ancestors in old photographs which unfortunately have been passed down without labels. We talked about clues found in two books, now available on CD from the publishers of Family Chronicle magazine. (See my Second Life avatar standing in front of the two book cover displays.)
We have to sometimes take into account that for instance a daguerreotype of a young woman probably dates between 1839 and 1850 when the ambrotype process came into vogue. Therefore the woman cannot possibly be your 2nd great-grandmother who wasn't born until 1871.
Clothing, the mounting board and the backdrop can also be clues. Ol' Myrt here advised my Second Life friend to use these two books (on CD) to reason out the possible identity of the subjects in her two family portraits.
Happy family tree climbing!
Your friend in genealogy.