I found something out the other day - when supermarkets have BOGOF deals etc, it's generally not the supermarket that is taking the hit, as normally they pass the majority of the price reduction on to the manufacturer by paying them less in return for shifting higher volumes. The supermarkets win either way of course.
Then there are the old tricks of putting things like eggs and milk right at the back of the store so you have to walk past all the stuff you don't need to get to them, etc etc!