(MONEY Magazine) – My daughter Julia and her friend Chelsea were eating pancakes at the kitchen counter one recent morning when the doorbell rang. It was the man from Peapod, an online grocery store, with my food for the week. "That's a lot of groceries," Chelsea said. "My mom shops for dinner every night."

She's not the only one. Grocery shopping seems simple enough, but Americans are wasting more money, food and time than ever by not planning. We spend more on food each year (an average of $5,340 these days) than on anything else besides our house and car. We research those two purchases exhaustively before buying because we know that the bigger the line item, the greater the opportunity to save. Can't we spend 10 minutes on a grocery list?

"Americans have forgotten how to food-shop," says Phil Lempert of SupermarketGuru.com, which tracks the industry. "When we don't plan, we buy the wrong things, which causes us to spend more money and more time."

Today households on average toss 14% of the food they buy, about double what we threw out 20 years ago. Compare this with our parents' and grandparents' generations, when time was spent each week planning menus so that every last item that was bought was used.

Indeed, today's meal planning goes more like this: It's 4 p.m. and you haven't thought about what's for dinner. You dream up something easy--baked ziti!--and run out for ingredients, forgetting that in the back of your Sub-Zero is a block of the same cheese you just bought, right next to last week's leftover ziti. The next day at 4 p.m., it's the same routine.

To meet our mothers' generation in the middle, we crave foods that are fast but that also have a homemade feel. (According to Nielsen, one of the top daytime shows on the Food Network is Semi-Homemade Cooking.) Food purveyors have been happy to oblige, preying on your lack of time and your seemingly endless grocery budget by concocting pricey ready-to-eat foods. "More shelf space is dedicated to prepared food these days," says Michael Sansolo of the Food Marketing Institute. "Soup comes in a grab-and-go cup. It wasn't long ago we didn't even have juice boxes." Some new products are lifesavers; others are rip-offs. All are part of a changing grocery landscape that includes everything from pre-diced onions to grocery lists you can keep online. To shop smart, you have to decide which alleged improvements really save time and money.

• Be picky. The premium on shortcut foods--marinated chicken breasts, cut vegetables, washed lettuce--is enormous, so compare prices of the prepared version and the normal version, then decide whether the premium is worth the time you'll save. Take lettuce. You can get a head of romaine for $1.99 or a bag of Earthgreens organic romaine hearts for $3.99. Would you pay someone $2 to chop and wash your lettuce? Not on a lazy Saturday. But on a frenetic weeknight it might be a bargain.

• Use what you have. "There are literally 150 pasta dishes that most people could make with stuff they have in their house right now," says Mark Bittman, whose New York Times column The Minimalist and book The Best Recipes in the World are aimed at today's frenzied shopper. Lempert suggests a weekly use-what-you-have night. Involve your kids. They'll eat whatever you cook up simply because they helped (trust me on this), and you won't spend $40 ordering in.

• Make lists. Half of us don't make shopping lists. That's why we buy bags of food but have nothing for dinner. Before you shop, plan your next three dinners, trying to pick ingredients that overlap from meal to meal. That way you won't buy something you'll use half of and then shove to the back of the fridge to compost.

• Shop online. Our mothers would have. Jodie and Lawrence Smoler, parents in Briarcliff Manor, N.Y., used to spend $250 a week at the grocery store. Once every few weeks they'd fill a hefty bag with everything they hadn't eaten--vegetables on the wrong side of ripe, cold cuts past their prime--and throw it out. Then they discovered Peapod, the online grocer in their area. Jodie's first foray onto the site took about an hour as she searched for her staples. But now every week she starts with that same list, adds a few necessities and is done in 10 minutes, for around $90 including delivery. "Last week she was away, so she didn't order," Lawrence says. "I went to the store instead. I spent $150 and I can't figure out why."

FANTASY AISLE Time-saving products are popping up on grocery shelves. Most cost more, so ask yourself if the time saved is worth the money.