Mark Thoma's thoughts on why savings have fallen are very interesting, but a bit economically deterministic for my tastes (and I do love my economic determinism!). He writes that "[s]aving didn't fall because we suddenly decided to party rather than work or because we forgot the value of a penny saved. There are fundamental economic forces at work generating these changes, low interest rates, high saving rates in other parts of the world relative to attractive investment opportunities, the forces of globalization, and so on." That's certainly true, but only in part.
I'd suggest that savings have indeed fallen because that sort of money management has been culturally de-emphasized. A cursory search didn't turn up historical polling on the issue, but anecdotally, my grandparents are much more invested in trying to sign me up for, among other things, life insurance, than anyone else in my life. This may mean they want to bump me off, or it could mean they take a longer view than I do. It's certainly true that the risk-and-reward approach to the economy -- wherein you gamble for great wealth -- has grown in importance relative to the visions of middle class security -- where you become a unionized machinist -- that animated the post-war economy, and I'd assume the decreased emphasis on savings is, at least in part, a function of that cultural shift.
Update: And as Matt mentions over IM, the fact that the Great Depression is well within my grandparent's memory while I've only known relative prosperity no doubt impacts our attitudes towards saving as well.
