Keeping your budget in mind is always part of a frugal lifestyle. There are many things you can do to make sure you don't go over budget on your shopping trip.
First, preplan your shopping trip by making a list of items you need, as well as setting a budget before you even enter the store. You are less likely to stray off course when an attractive item is dangling in front of you.
Second, just because it is on sale, doesn't mean you should buy it. Many shoppers are drawn to sales, but most don't realize that 1. The item may not actually be on sale (the store raises the price, then "pretends" it's on sale) and 2. If the sale item is not on your list, you are going to go over budget and 3. If you have to buy a sale item, buy it because it's something you would normally buy, and not just because it's on sale.
Third, know the prices of the items you buy regularly. I'm pretty good at keeping a mental note of the general cost of the items I buy but if you want to get super-accurate about your shopping cost, keep track of how much the things you buy are, that way you will know which store has the best deal on that item.
Many more great shopping tips to come!