From the Wall Street Journal:
WSJ: What are some of the biggest things people waste money on?
DR. SHILLER: Big houses are a waste. People are still in a mode of thinking about houses that is kind of 19th century. As we modernize, we don't need all this space. For example, we don't need elaborate kitchens, because we have all kinds of delivery services for food. And maybe you don't need a workshop in your basement, either. You used to have a filing cabinet for your tax information, but now it's all electronic, so you don't need that, either. And bookshelves, for people who read a lot. We have electronic books now, so we don't need bookshelves anymore.
Subscriptions to health clubs are notorious, and capitalize on people's failure to anticipate their future laziness.
MR. BACH: Small-dollar purchases like coffee and bottled water. When you ask people why they don't save, why they don't invest, why they don't use their 401(k) plans, the No. 1 reason is "I don't make enough money." Five dollars a day makes a huge difference. If you're sipping a latte right now and you're not saving, well, that's dumb.
MS. WADDELL: The primary thing I see people waste money on is "convenience" items and unplanned grocery-store trips. Lack of planning leads to people stopping to pick up one or two things and then spending $100 or more.
DR. BRICKER: It's often in quickly consumable items – mostly things you can put in your mouth, so convenience foods, sweet foods, and things you can drink. You can consume them immediately, so there's an immediate reward that you get from it. And you are not seeing the longer-term perspective on how that consumption is not only harmful to your wallet but also harmful to your long-term values – what we want to be doing with our life.