An addendum to rule #2 (only shopping from shopping lists, no impulse shopping) as follows:
If, when shopping from a list, I notice something that I forgot to put on the shopping list, it is permissable to pick up this item.
For instance, today I went to the grocery store way out in the boonies because the closer one didn't have the thing I needed to make the recipe I am making, and when I was there I walked in and saw tomatoes. "That's right," I said to myself. "I just used up my tomato yesterday, preparing a frugal, homemade sandwich for my lunch. And they are on sale! Ergo, I will buy one tomato, though it is not on my list."
This is OK. Because I say so. And it would be silly to go all the way out there, write "tomato" on a list for my next shopping list, and not have a tomato to make into a sandwich for this weekend.