What I've been doing is making a shopping list at Wegmans.com of things we need, even though I know I'll probably buy things on the list cheaper at Save-A-Lot or Aldis. I figure if I can keep my list in the vicinity of my budget then buying things should bring me under budget or allow me some extras. I keep track of how much less money I'm spending on the margins of my list. So if I have $2.00/lb for chicken breasts on my list but find it for $1.70/lb and I buy 5lbs I'd write + $1.50 in the margin. Then if I buy something extra I cross off the margin amounts to total the price of the extra thing. It's been working out well. I love that I don't have to keep a running total in my head when I'm going through the store. I do have to remind myself that I can always come back another day and pick up more extras if I come in under budget at the end of the shopping trip. And also to leave some $$ for restocking perishables since I have to go 2wks between major shopping trips.
So hopefully on Monday (won't be home this weekend) I can come back here and let you know how things went.