1. Never go when you're hungry
Look, never make a trip to Walmart when you're hungry. Just don't do it. That's extremely dangerous territory - you'll go in for bread and milk and come out with pizza rolls, Twinkies, six frozen dinners, a tub of popcorn chicken from the deli, and a 16oz bottle of regret.
2. Make a list
The magical Walmart gods are smart people. They know how to strategically place all of the products in their stores so that as you make your rounds through the aisles, literally nothing you intended to buy catches your eye. Toothpaste, I need toothpaste - but oh look at this cool little thing that squeezes all of toothpaste to the top of the tube and woah does this mouthwash really whiten your teeth that well? And next thing you know, you just walked out of the store with a $30 water dental jet?????
3. Keep a budget
I cannot tell you how many times I've gone into Walmart intending to only spend about $10 - $15 and walked away with a receipt that added up to $40 or more...MAKE A BUDGET. And stick to it. You'll be happier and so will your bank account. And just think, with all the money you save you'll be able to make another trip to Walmart :D