I started doing this unintentionally because of my two least favorite things about food: not having the ingredients I needed for a meal I was making and going to the grocery store! My dislike for both of these things is why I would buy several of the things I knew we would use when I found them for a good price. Which goes along with my next point.
~Buying in bulk
We've always bought our meats in bulk, usually from Sam's Club because we noticed a savings per pound that way. It also was convenient to have meat in the freezer to simply pull out for a meal. I did the same thing with pasta, rice and other items that we found to be cheaper and nice to have in large amounts on hand.
Then, I discovered that this method of shopping is actually a common practice for the frugal minded and something that made the grocery shrink so popular. So I've taken my philosophy to a new level and started stocking up on veggies through out the summer and other things I hadn't thought to stock up on as often (like homeade freezer meals). This process has been the number one way I've kept our budget so low because I buy meat about once a year and other items much less frequently as well, so on a monthly basis I have very few things I need to buy to make a meal (usually just the fresh/perishable ingredients). The rest of the budget is going to continuing the stock pile.
The bonus is that I don't go to the store very often either! :)
Like I've said before I take in all the local ads and get the best price in town for the items I need! :) Knowing the prices for the things you buy is the best way to know if it's actually a deal or not, so memorize them or keep a book to log your local prices. Then, while looking through the ads you'll be able to quickly identify which prices are actually a "deal" or "sale" and which are still more expensive then the prices at other stores.
~Buying the cheapest brand
I'm not loyal to any brands, so I always get the cheapest brand (per unit price) on nearly everything. If there are things that we prefer a specific type of--like our bread--then I'll buy a ton of it when it's on sale and freeze it. Otherwise, we usually buy generic, unless there is an occasional sale or coupon that makes the name brands less expensive.
~Buy what you'll use
Especially when it comes to perishables. If you can't freeze it or can it before it goes bad, then don't over buy it, even when it's a good price. I usually buy just one or two vegetables for the week and then rely on frozen or canned produce we've stocked away to fill out the week's meals if necessary. Then I always keep things like apples or oranges on hand because they last much longer then other fruit. Then, when I find other options on sale, I'll buy only enough for a few days and resort back to the apples/oranges for more fruit until the next visit to the store.
~Avoid convenience items/individual packaging
I've mentioned this before--a lot! This has to be one of the best ways to keep your grocery budget lower. Rather then paying more for those items at the store, I make my own convenience items. I know that for someone working a full time job, those items are almost necessary, but I'd encourage you to find ways to minimize them as much as possible. Even when I worked full time, I would get the large packages of yogurt and pour individual servings into containers for my lunch. I found that the larger container would get me more servings as well. I do buy some convenience items, but I try to keep that to a few. By convenience items I'm referring to pre-made meals or frozen dinner type things. It's usually significantly more expensive then making those meals from scratch yourself.
The end result is more money left in your hand and plenty of great food to feed your family! :)