Your Price Book
People who buy things for a living keep track of prices carefully, noting who has the best deals when. You can apply the same logic to your grocery shopping (and anything else you buy regularly).
The government has published some guidelines for what people spend on groceries. But food prices vary tremendously in different parts of the country. If you keep track of what prices you see for your normal food items for a few months, you will be amazed. If you can get to the point where you are only buying things when they are at good prices, you will lower your grocery bill with very little effort. For example, I regularly see a 5 pound bag of all purpose flour for over $2 where I live, but right now they are on sale for .69 in a few places. And watch the "buy one get one free" ads: often they're not much of a deal.
Your price book can be as simple or as elaborate as you'd like. I have done it on paper before, just using a small notebook with a page or half a page for each item. I've also started doing it in Excel, which has the advantage of calculating the unit cost for you automatically once you've set it up but needs to be printed and carried around when you go shopping.
Click the image to enlarge. Click here to open the spreadsheet if you have Excel.
Start by grabbing your last few weeks' grocery bills.
If you live in an area where using coupons makes sense, ask friends for extra coupons or newspapers--especially when you see a good coupon. Some magazines have good coupons, too. I've been sent a few copies of All You magazine and was impressed enough to sign up as an affiliate (meaning I'll make a small commission if you click on the following link to subscribe). I wouldn't recommend coupons normally but the ones in here have been quite good. There are a lot of things I never buy but there are always a few useful ones. And typically there's way more than enough to pay for the magazine, which also has tips and articles on cooking and saving money.