There are a lot of stores out there, and they want your business. Or more precisely, they want your money. Preferably, as much as you are willing to give them, and then some you wouldn't willing give up. In order to get as much of your money as possible, they're going to do everything they can to get you to spend more money, even on things you don't need, or even want.
Now, I suppose we can't put too much blame on the stores (and more to the point, on the people who run them); they are, after all, just trying to make the maximum amount of money possible, something we all want to do. However, that shouldn't stop us from learning about these tricks, figuring out how to resist them, and keeping as much of our money as possible. I'm sure they won't blame us for that (and if so, well, too bad). In any event, let's look at:
5 Tricks Stores Use So You Spend More Money
1. Confusing Pricing: How often do you see stores use ‘$4.99' rather than ‘$5' when listing the price of an item? When you do, you probably tell yourself that they aren't fooling anyone, because ‘everyone knows that it's basically just $5'? In spite of that understanding, though, it's easy to be taken in; our brains subconsciously reads $4.99 as ‘four-something', compared to $5 as ‘five-something' , regardless of how much we tell ourselves we aren't being fooled. (Our brain also tends to view items with a price that ends in 9 as ‘on sale', even though saving a penny is hardly much of a bargain).
That's just the start. For any given item, you're trying to compare a wide variety of different producers, sizes, and varieties, all selling at different prices. Trying to determine whether the 44.2 ounce container selling at $9.99 is a better deal than the 26.3 ounce container at $5.29 requires a lot more math than most of us can do in our heads, particularly if we have to buy another thirty items before we finish our shopping. This is even before we get into how sales can affect these prices (but we'll get to that).
How to Avoid It: Ladies and Gentlemen, the unit price:

The unit price shows the price for a given unit of each item. In this case, it's showing the price for a pound of tomato sauce; for a liquid, it would be per fluid ounce and for something like paper plates, it would be per 100 plates. It makes it easy to compare the actual items on an even basis, rather than trying to estimate which is the better or worse deal, or having to break out a calculator for every item that you buy. It's probably still a good idea to have a calculator (or phone with a calculator function) for those times when things get even trickier, such as:
2. With ‘Sales' That Don't Save You Anything…Or Cause You To Spend More: Hey, a sale is a good thing, right? Well, if it's saving you money on something you need, then sure. But quite often, the ‘sales' price isn't much of a deal, and can actually be worse. From giving you 10% off a price that is 20% higher than normal to selling items as ‘2 for $5!' when each normally costs $2.49 (there's that 9 again), it can be easy to see that SALE sign and just assume that the price is lower than normal.
Even when you are saving money with a sale, you may end up paying more than you would otherwise. If the sale is promoted as ‘5 for $5', for example, you can end up buying five of that item just to get the better price, even though most places will ring up each item as $1, regardless of how many you have. There is also a bad tendency for people to buy things that are on sale that they don't need; even if everything in your shopping basket is 90% off, if you don't need it, you'd be better off just leaving it and keeping your cash.
How to Avoid It: Breaking out that calculator and doing some quick calculations is a good way to start. If you find that the sale doesn't really save any money, still leaves the product more expensive than an alternative, or ends up making the better buy into a worse one, you can avoid the sales item and go with an alternative (assuming you actually need the item, of course). As for when you come across sales that ‘require' you to purchase 2, 3, or more items to get the savings, get only as many as you need and make sure to find out if you will get the reduced price anyway; if so, why buy more? (And if not, it might be a good time to look for a new store…) Regardless of which sales are available, though, you have to make sure that you only buy what you need so you don't spend more than necessary.
3. The Store Layout Induces Shopping: The longer the store can keep you shopping, the further you have to travel to get your items, and the more you stop and look around, the more you'll buy and the more the store will make. There are numerous ways they attempt to accomplish this. Staple items (your milk and bread; literally, in the case of supermarkets) are placed far from the entrance and the checkout, causing you to walk through the entire store, usually twice, even for a quick stop. Aisles are small, limiting how many people can get through; while you're waiting, it's all the easier to see something you'll end up buying. And of course, the impulse buy items are right by the checkout area, ready to be seen and bought (sometimes they even cost more than equivalent sized items found elsewhere in the store).
How to Avoid: Being aware of these methods is a good start, as they will have less effect if you know why, for example, you have to spend five minutes walking just to get some milk. Another useful technique is to plot out your course through the store, so that you can try to minimize the time you are inside the store, and limit the temptation you encounter. Even so, you'll still see:
4. Eye-Catching Displays and Slick Packaging: We're surrounded by advertising nowadays, and inside stores is no different. Items are placed so that you are led to the most profitable items and purchase them as readily as possible. The end caps feature desirable items (sometimes on sale, but often just at their regular prices) to draw people into the aisles. The most expensive items are placed at eye level (or your child's eye level, for the sweet foods and other child-friendly items), where they can catch the most views. The packages themselves are designed to draw attention, with the brand name items being brighter and more eye-catching than the generic or brand name versions.
How to Avoid: Getting blinders from your local horse trainer might work. If that seems too awkward, a better method might be simply make a list and stick to it. The whole point of these techniques are to make you spend money you didn't intend to spend, so make sure that you buy ONLY what is on your list, and if you come across something that you MUST have, write it down and consider it carefully before buying. (Ideally, go home and wait until your next trip, when you've had several days to think about whether you need the item or simply want it, but at least give it your consideration for a few minutes or longer as you through the rest of your list.)
5. Using Lights and Sounds to Induce Purchases: As long as you can see and hear, the lighting on the displays and the music in the background can have a major impact on what you end up purchasing. From using bright lights to imply freshness to providing music that speeds you up or slows you down (depending on how quickly they want to get you out of the way of other shoppers), there are quite a few techniques that stores can use to influence your shopping that you might not even notice if you don't pay attention.
How to Avoid: The music part can be dodged by providing your own soundtrack: bring in headphones and put on your own music, preferably something that will keep you moving and shopping in a rational manner. The lighting is trickier, as trying to do things like read a list and comparison shop require you to be able to see, preventing the use of thick sunglasses. Instead, just try to be aware that the lighting you see may not be purely there to ensure that you can read what is on the package, and might have an ulterior motive.