Just because it is on sale or on a promotion doesn't mean it is a great price. For example, take a look at a snippet from this week's Smith's ad:
Alright, Stephanie. So how do I know which deals are good and which ones aren't? Honestly, the best advice I can give is to pay attention to the price of items you frequently buy. I keep a mental note of what a "good price" is. During the summer I bought a lot of chili without coupons and I knew it was $1.50+ per can. When I saw it on sale for $.99, I knew that was a better price and that I should stock up on a few more cans. Then, when I saw the coupons to go along with it, I knew I had better really stock up! For another example, I will not pay more than $1.00/box on cereal because I know I can always get it for that price. I may have to wait awhile, but there will always be a cereal promotion.
Retail and spending in general is all about what you are willing to pay. You have to decide what that amount is.