Know How Much Things Cost
Sometimes I see the Sunday paper and realize something like diapers are on sale. (And just for the record, we primarily use cloth, but Saw's school and our church nursery require disposables.) They're three dollars off, and I have a coupon, so that means they're only $6! I could run over to Kroger and pick them up...or, I could realize that they're still 50 cents cheaper than that at Walmart without a coupon. Or, try this for size: Kroger has a 10/$10 sale (remember you don't actually have to buy 10!) of canned tomatoes. I see the yellow sale tag and stock my cart. Then realize the next week that when they are not on "sale," these cans are only 89 cents. Doh!
This is why it's important to know how much you should (or can) pay for things before you hit the store. You don't want to stockpile or even buy an item if it's not as cheap as it could be (unless it's a must-have for the week and you have no choice), even if it's on sale and you have a coupon. One way to think about this is to know the costs of things on a daily basis where you shop, both the name and off-brands. After you've been doing this for a while, you'll recognize how much or little you can get away with paying for things.
I have realized that it's possible for me to get toothpaste for as little as 25 cents or free. So I will not pay more than a quarter for a tube of toothpaste (which, by the way, is my favorite Crest whitening). Recently, a sale plus coupon made it free, so I stocked up. Toilet paper and papers towels in 12+ packs should be under $5 or I won't buy.
An exception might be if you're buying at CVS or Walgreens using their gift certs (ECB and Register Rewards, respectively). It might really help you one week to spend more, but use your ECBs you've stored up, meaning it won't cost out of pocket. Also, if you pay a little more out of pocket to earn ECBs to use the next week, it might even out. If you need a certain item that's not on sale that week, then you might just want to buy it wherever it's cheapest and whatever brand. Here's a link on Southern Savers about how to work with a buy price list. I don't have a great buy list yet, so here is another link from Southern Savers and some suggestions from the class I took in May: