1. Follow the Time Rule
I've seen this defined as something as small as the "one hour rule" and as big as the "30 day rule," but the idea is the same — when you see something that you want, make yourself wait a certain amount of time before purchasing it. The longer you can go, the better. If you still strongly want to make the purchase at the end of the time period, only consider doing so then.
2. Don't Shop When Upset
It's easy to look for a product (whether it's food, clothing, or something else) to cheer you up when you're unhappy. One of my worst impulse purchases ever happened at a liquor store — and in an effort to cheer myself up, I purchased three kinds of liqueurs I had been wanting to add to my bar.
3. Consider Changing How and Where You Shop
One of the things I love about shopping online is that it's much easier to ignore extraneous items — I go to Amazon, put what I need in my cart, and check out. But I know if I try on clothing in a store, I'm much more likely to happen upon a dress that I suddenly really want. The solution? Except for groceries and pharmacy items finds, I rarely shop in physical stores.
4. Don't Shop With the Wrong People
If you have impulse-happy shopping buddies, it can be easy to let them convince you that all the outfits you just tried on look great, and you should TOTALLY buy them. If you want to shop socially, do it with people who have also frugal spending habits (and if they happen to be able to tell you when a dress really looks great on you, well, all the better).
5. Give Yourself a Splurge Budget
Whether it's budgeting for one new clothing item a month, allowing yourself a fancy coffee every now and again, or giving yourself spend-it-however-you-want cash, give yourself some room so you don't feel like a penny-pinching miser.
6. Only Buy Things You Can Return
If you really have a problem with impulse purchases, at the very least, buy from stores with good return policies.
7. Remember to Not Be Fooled by Sales
Sales with huge markdowns can make impulse purchases very tempting. For years, half my shoe collection was made up of shoes I only sort of liked, but had found on sale. Remember, if you see a product on sale, you will always save more money if you don't buy it at all.
8. Keep a List of Things You Really Want or Need
That way, if you do see them on sale, you can buy them with confidence.
9. Don't Give Yourself Access to Your Money
Whether it's leaving your credit cards at home or only carrying a certain amount of cash, you can't make impulse purchases if you don't have the money to do so.