ONe of the most proven methods I've used (in addition to the ones listed here) is to shop just ONCE a week. That's it - no stopping in for a gallon of milk, loaf of bread, etc. If we run out, it's jsut too bad and we wait until the next week. This has made me be more contentrated on my shopping list and planning meals ahead.
We're so used to just popping into the store, sometimes a convenience store with higher prices, to grab a carton of milk or head of lettuce, that we don't think about it. I've suspected one of the reasons I spend so much on groceries is because I do this. I'm going to be more hardcore about not doing it from now on.