No matter how much you trim your budget, one expense that's sure to remain is food. But how much you spend on groceries each month is not fixed, and can vary by 15-20 percent or more. So because the grocery stores consider it their job to get you to spend more money, it is your job to keep your wits about you to avoid getting ripped off (especially on those grocery trips when you just wanted a gallon of milk, and left with five bags full of impulse buys).

It's surprisingly easy to save a significant amount of money on your food bill (even up to $100 a month). Here's how.

Check your receipt. Scanned items do not always ring up at the correct price. Watch the prices as they come up, and then check your receipt for those you missed, to be sure sale and other items come up at the right price and quantities are correct.

"I can't say it's widespread, but I do get reports of people saying they check grocery bills, and very often they find mistakes ... And, 4 to 1, they are in favor of the store," says Gary Foreman, publisher of "The Dollar Stretcher" Web site.

Resist Impulse Buys. Grocery stores purposely set up enticing displays of expensive cookies, crackers and other packaged foods hoping to coax you into buying something on a whim. Making a list and sticking to it will help you to avoid such impulse buys.

Also be aware that grocery stores purposely place common items all the way in the back of the store, so you have to walk down the aisles to get there. The more snacks, cereals and other processed foods you're exposed to, the greater the chances that you'll grab something not on your list. One way around this is to only shop at the outer perimeter of the store. This is where you'll generally find the lower priced, and healthier, items like meat, produce and dairy. Processed foods are generally on the interior.

"Organize the shopping list to match the layout of the store to minimize time needed to retrace steps and to decrease impulse purchases," said Mary Meck Higgins, Kansas State University Research and Extension nutrition specialist.

Don't Shop Hungry. Even with the best intentions, it will be hard not to over-buy when you grocery shop on an empty stomach. Grocery stores know this and may entice you further with the smell of fresh bakery items and breads. Try to eat something before you go, or at least drink some water.

" ... Purchases often are impulse buys that can add to the grocery bill unnecessarily," said Higgins. "Impulse purchases also can increase when shoppers are hungry or shop without a list."

Look at the whole shelf. Stores place the items they want you to buy most--the ones that give them the most profit--where they're easiest for you to see: between knee- and shoulder-height.

"The highest markup items are the ones at about chest level--to make it really easy for you to grab it and toss it in the cart," says Foreman.

Be sure to look at products on the upper and lower shelves, too. This is where you're likely to find a good product at a lower price.

"The old saying, 'look high or low to save dough' still fits," said Higgins.

Buy generic. Often, generic products contain the same ingredients as name-brand versions at a fraction of the price. They may even have come from the same factory. All you need to do is compare the nutrition information on packages. If the ingredients are the same, the product probably is as well. Do watch out for extra additives or imitation ingredients, though--some generic versions, for instance Parmesan cheese in a carton, may contain lower quality ingredients or more additives than the real thing.

Make sure sales will save you money. Don't assume that just because something is on sale (or comes in a "bulk" container) it's a good deal. Sometimes the generic version or a different sized container will be a better deal than the sale item. Check the unit price (usually found right on the shelf's price label) of each item to find out how much you're actually paying. And, if the item turns out to be a bargain but the store has run out, you can, and should, request a rain check to use on your next visit.

Only buy groceries. Grocery stores are best saved only for groceries. Toiletries like toothpaste, shaving supplies and soap and other health and beauty products are typically significantly cheaper at drugstore chains than at grocery stores.

Find a store that matches prices. Some grocery stores will match competitors' prices, though they may not advertise this practice. "Find a store in your area that will honor all competitors' ads," says Ellie Kay, author of "Shop, Save, and Share."You'll save money, time and gas."