Meal planning helps you stick to what you need and allows it to stretch. A few people said to plan meals for the week, one or two big meals that can be leftovers for the next day or two, and smaller meals with little to no leftovers.
Stores usually put the most expensive products at eye level. Even brands stock their cheaper fare lower to tempt you to buy their pricier products, someone else added. So make sure to check out those top and bottom shelves.
Though I've heard of this one, I've never remembered to use it. If you find an advertisement for something you want at your grocery store's competitor, and it's cheaper, you can ask your supermarket to match the price.
The one countless shoppers mentioned, but few of us can do, is stick to a list. You know your grocery store's layout, so several commenters mentioned not just making a list and sticking to it but writing the aisles that house the products.