Summarize this article in one sentence.
The biggest way people end up overspending at the grocery store is inadequate planning. Before you go to the supermarket, take stock of what you already have.  According to a study by the National Resource Defense Council, Americans typically end up throwing out 25% of the groceries they buy. You can prevent this wastefulness by taking stock of your pantry and fridge before making a trip to the grocery store.  Find items that are about to expire and move them towards the front of the fridge or pantry. Make a note on your calendar or in your kitchen to use these items for meals in the next few days. Do this at least once a week to prevent wastefulness and save money on food. You should always make a list before you go shopping. Impulse buys can add up over time, resulting in you spending a lot more than you intended. You should also make sure to be careful when you shop. Avoid shopping hungry or stressed and review purchases when the shopping trip ends.  In addition to making a list, try to schedule you're shopping as well. Aim to only shop once a week, as this encourages you to use the food you have rather than going out and buying more unnecessarily.  Review your receipt when you're doing shopping and circle the items you spend the most money on. Consider what you could substitute for certain pricey items and what items you could go without. The day of the week you go shopping can actually make a difference. Aim to shop mid-week, preferably on a Wednesday.  Food tends to reach its "sell-by" date midweek, so you're likely to find items marked down on Wednesdays or Thursdays.  Stores also add weekly deals and Wednesdays, but are also likely to honor the previous week's deals. This means Wednesdays are a great day to try to double up on coupons or sales. When planning your weekly meals, do so around what items are on sale.  It might be a good idea to make a list around sales prices and then meal plan. That way, you will not be tempted to include unnecessary items that are not marked down to complete a recipe. You will have to work with what you have.  You can also shop and then meal plan. Many websites allow you to type in a list of ingredients you have and then offer you a variety of recipes you can use. Some items are cheaper if you buy them in bulk. If there's anything that won't go bad too quickly, see if the store offers specials if you buy in bulk. Make sure you're really getting a deal, however. Many bulk prices actually come out to about how much you would spend on an individual item. Do a quick calculation in your head or on your phone to make sure buying multiple items is actually cheaper. Some stores will offer a small discount if you buy an entire case of a product.

Summary:
Take careful stock of your pantry and fridge before shopping. Make a list and shop carefully. Shop on the right days. Plan meals around sales items. Buy in bulk.