my best friend and I were discussing the other day that when shopping for produce its much cheaper (and more delicious!) to shop seasonally- citrus and root vegetables in the winter, greens and such in the spring, berries and tomatoes and such in the summer and lots of tree fruits and vegetables in the fall. If you've ever noticed, prices fluxuate throughout the year on most produce.
reading the shopping inserts in the papers usually give you an idea as to what the costs of things are at the local stores, so that you can shop for deals as opposed to just shop for everything at one store. I'll usually hit a few stores when out shopping for grocieries in order to get the best price and quality on everything (of course, I live within half a mile of about 10 stores, which helps)
ethnic grocieries are great places to buy cheap spices, rice, beans, noodles and a lot of more exotic fruits or vegetables ($0.50 pomegranates?!) And if you cook something other than the traditional meat and potatoes type meals you'll use meat in smaller portions- which is definitley cheaper
I've lived on less than $50 in groceries a week following those rules and ate quite well! (of course now I have a weekly grociery budget of closer to $125, so the food is awesome )