I've been using a price book and really keeping track of prices for about a year and a half now. It has really paid off. Yesterday was a good example.
I go grocery shopping every two weeks. Between trips I write down things as I think of them, as they come up, as we run out. On Wednesday, I took my list and put it down on a sheet of paper. I list each store I go to then what I need to buy there. For the first time, I went through and estimated how much all the items would cost and I wrote this down next to the store name.
For Fred Meyer, I had coupons for toilet paper and english muffins. I also found a dollar off in store coupon for tampons I used. I bought everything on my list and the only two things I bought that weren't on the list was a head of lettuce and a large bunch of spinach. My estimate was $50. My total: $50.00!! :)
I was also on target for most of the rest. Wal-mart estimate $10, actual $10.31. New Seasons Market estimate $15 actual $15.63. Trader Joe's surprised me. The automatic dish washing detergent I use (along with washing soda and borax) was WAY cheaper then the Seventh Generation one I bought about a year ago. I estimated $30, actual was $20.
Of course, this works if you only buy what's on your list. Other than the lettuce and spinach, I did not buy anything not on my list.
I think I'm finally getting the hang of this frugality thing! :)