I figured out how to feed my family of 4 on $120 a week with these key sales hacks

• When my son was a baby, I overdraft our account by buying a $5.99 sweater.
• Since then, I've been budgeting and trying to minimize our spending.
• I spend $120 on groceries by buying seasonal sales and starting around the perimeter.

In one of my most potent memories of early motherhood, I cry while clutching a tiny argyle sweater I bought for my infant son. At $5.99, it seemed an irresistible deal, a chance to hilariously dress my 10-month-old like a baby professor. Who can blame me?

My bank, apparently. I'd bought the spontaneous sweater the same day I went grocery shopping, and my faulty math and meager funds had caused me to overdraft. Things had been tight since my husband and I had gone down to one income so I could stay home with our son. Student loans, medical bills, and rent left little behind for groceries and gas.

The day I bought that sweater, I learned an important lesson. I knew I had to teach myself some tricks of the grocery store that would save my family money and allow for occasional impulse buys. Even though I work full time now and have a roomier budget, I still use the same shopping habits I developed 13 years ago. These days, on average, I spend around $120 on groceries every week.

I pay attention to the sales cycle

The trick is hacking the grocery store sales cycle — or knowing when to buy items when they are at their cheapest.

Goods are sold in a pattern called the sales cycle. A savvy shopper will become familiar with that pattern to buy at the most opportune times. Whether you realize it or not, you're already familiar with these patterns as a buyer: Early Bird Sales introduce a buyer to a new product, whereas Clearance Sales move the product off the shelf to make room for new merchandise. These are examples of sales cycles.

Groceries, which must be rotated more frequently than other goods, have an even faster and more consistent sales cycle. Planning to buy something in bulk when it's on sale will help you save money.

Many foods are on a six-week sales cycle. If cans of tomato sauce are on sale this week, instead of buying a week's worth, buy six, and don't buy canned tomatoes again until they're back on sale. This is true for most shelf-stable grocery items, including condiments, sauces, cereals, snacks, and baking items.

Foods that perish faster, like produce and meat, have even shorter patterns. Many grocery stores are on a three-week meat cycle. Chicken, ground beef, and pork go on sale in a three-week rotation, so instead of buying all three of those items in one grocery shopping trip, I spend our entire meat budget on chicken the week it is on sale. Then, I do the same for ground beef and pork. I package it in smaller batches when I get home from the grocery store, freeze it and then take the meat out to thaw the morning I plan to use it.

I tend to be unwilling to pay more than $3 per pound for meat at my rural Montana grocery store because that is the sale price. I buy about 10 to 15 pounds of whatever kind of meat is on sale that week, and my freezer is well used.

Produce is on a similar rotation, although it varies from store to store. I typically don't spend more than $2.50 per pound on fruits and vegetables.

I recommend watching the sales ads to learn your store's sales pattern, and don't be afraid to ask questions. The people who work at your grocery store are there to help you. The meat department manager can tell you when to expect certain types of meat on sale, and you can ask the produce manager about their rotation.

Here are other ways I make sure to stay under budget

Shop the perimeter of the store first. When grocery shopping, consider the necessities first: Produce, meat, and dairy. Once you've nabbed those items, you'll know how much money you have left for pantry staples and splurges.

Pay attention to other seasonal sales. For example, the day after Valentine's Day is an excellent time to stock up on chocolate. I use it by chopping it into chunks and adding it to cookies and pastries.

Learn how to use one food for multiple purposes. Did you know you can substitute unflavored full-fat Greek Yogurt for sour cream or applesauce for oil in baking? These are not only healthier substitutions; they also reduce the number of items you need to buy at the grocery store.

At $6/dozen, eggs are really beginning to seem like a luxury for any budget. I still make eggs on the stove, but when I'm baking, I take a note from my vegan friends and make a flax egg. Mix 1 tablespoon flaxseed meal (ground flaxseed) with 2 1/2 tablespoons of water. Let it set for 5 minutes to thicken. Replace with one egg in baking.

Don't miss markdowns. Like any retail store, grocery stores need items to move off the shelf on clearance. Veggies just past their prime make great soups or roasted dishes. Fruits can be frozen to use later for smoothies or baked goods. Breads and most baked goods also freeze well and can be warmed up for meals at a moment's notice.

Shopping in these rhythms builds up your pantry and freezer. Buying items at their lowest possible price quickly turns into a fun competition with yourself. Congrats, you've just gamified grocery shopping.