Most stores lure you with great prices on a limited number of products. Sometimes these prices are true deals; sometimes they're rip-offs in disguise. Stores hope that when you come in, you'll end up purchasing things that have higher mark ups.
To get the best value from a trip to a big box or grocery store, before you head to the checkout, look in your cart for the ratio of sale to non-sale items. Was the store successful at bringing you in for sale items and getting you to purchase non-sale things?
In all cases, the best value will come from the final sober thought before you purchase something: will I need this? Am I over-paying? Even if you have to stop a debit transaction to remove an item, the inconvenience is worthwhile.