Know your prices.
Is it really a good deal? Don't buy 5 boxes just because the store tells you it is a good deal. Often the sales prices listed in flyers aren't anywhere near the lowest price you can find. Check out my weekly food deals posts, and think about keeping your own price book.
Find a storage place.
We are lucky because we have a big cold cellar under the stairs full of shelves and ready to hold all our canned goods, shelving in the furnace room and a big chest freezer. You may not have my optimal storage situation, but you almost certainly have some storage!
In our first apartment we had a small chest freezer right in the kitchen that acted as extra counter space. We still hadn't acquired much in the way of kitchen clutter and had room in the cupboards for extra food.
Keep in mind your pantry doesn't have to be in the kitchen. You will go shopping in your pantry as needed. A linen closet could work well, if you get rid of the old sheets you don't need and the ratty old towels. I've heard of people storing extra food under beds. Be creative!
Try before you stock up.
If you have found something that is a great deal, but you aren't certain that you will enjoy that brand or flavour, buy one and try it. Then stock up. We once stocked up on a certain brand of canned peaches I had never tried before. Yuck. They tasted like cardboard. Be warned. Make sure you will eat it!
Only buy food you actually use.
Don't stock up on cheap tuna if you don't like tuna! It isn't a deal if you won't eat it.
Organize your pantry.
I have a section for tomato products, a section for jams and spreads, a section for beans, a section for fruit, a section for canned fish, etc. You need to stay organized so you can find your food. It is great to get a good deal on canned peaches, but if you keep getting good deals and then loosing them in the pantry, you aren't making best use of your grocery dollar.
Keep the oldest food up front.
Just like at the grocery store, put the new food at the back, and the older food at the front, so it gets used first.
Watch best before dates.
Even cans often have best before dates. Make sure you can use what you buy before the best before date.
Try not to pass that line from stocking up to hoarding.
It is really easy to go crazy when you find a good deal. When something you use all the time is on sale for a great price it is easy to go crazy and buy, buy, buy! Keep in mind that most sales will come back again. Grocery stores have cycles and many things go on sale again and again.
Keep in mind the limits of your storage space. You want to be able to stock up on a range on things you use. If your space is all filled up with tomatoes, where will you put the baked beans?
Remember your budget.
When you are buying the food you need to eat this week, you probably find you spend a similar amount from week to week. When you are stocking up, some weeks you won't find anything to buy except your basic perishables. Other weeks there will be several great deals to stock up on. Your goal is to save money, so make sure you are saving money! You may need to adjust to a more monthly view of your food budget, rather than a weekly budget. If you used to spend $150 a week, you could start by making your goal to reduce that by $50 a week, with the plan that you won't spend more than $400 over the course of a month ($100 X 4 weeks).
You figure out what you can afford and stick to that. You don't want to over spend in your efforts to save money!
You might also be interested in reading about how I decide what to stock up on!