To keep from impulse-buying, I'll put an item on hold and give myself 48 hours to think about it. Majority of the time if I don't absolutely love it I forget about it. If I can't stop obsessing I know it's worth it.
Of course thrift shopping is my go store! This list would be incomplete without this trick. The pieces I find there are inexpensive, so I don't feel bad about taking a risk. The biggest plus of shopping at a thrift store is you'll never be at a party wearing the same thing as someone else!
I make sure that I can think of three things already in my closet that will go with what I am about to buy.
Before buying something, ask yourself these questions: Can I live without this? Will I wear this once or twice? Do I already own something like this? If the answer is 'no' to all three, then get it!
It's ok to spend on classic bags and shoes that you know you will have forever. But for the season's trendiest pieces shop at less expensive sites or stores like Asos, Zara, and H&M.
Last but not least, check that the return policy is fair and keep the receipt. If you haven't worn something by the time the return policy is about to expire, take it back instead of letting it just sit in the closet. I also like to sign up for emails from my fave stores. It helps you to be the first to know about sales, so you can get the best deals.