Ambers lists characteristics:
A large number of people who won't say who they want to vote for really just want to pick the winner. (This is THE reason why Mitt Romney's strategy includes a lot of braggadocio.) The late-deciders are disproportionately women. There are proportionately fewer of them each cycle, probably a consequence of ideological polarization and sorting. Some of them are undecided because they expose themselves to a diverse array of information, which is a good thing.
The Dish 