This one little tool: A hand-held row counter. (Mine is this one by Clover.) In knitting, you click it at the end of every row, and it helps you keeps track of your work so you stay on pattern. But after buying it, I realized it was just what I needed for grocery shopping! I keep it in my pocket and when I toss an item in my basket, I round the price up to the nearest dollar and click my counter.
Package of English muffins? $1.79 rounds up to $2. Two clicks on my counter. Organic eggs: $3. Three clicks on my counter. With this handy helper, I can estimate my total before I head to the checkout counter. Works great for me, because I walk into the store with a firm budget.
If I have a few “splurge purchases” I’m not sure I can afford, I keep them separated in the seat of my grocery cart. Once I see my estimated total, I know whether I can buy those items. All before I hit the checkout counter!