Most stores would like you to think that impulse buying is limited to sodas and candy at the checkout. In truth, impulse buying is anything you weren't planning on buying already. If you want to know whether the thing you're about to spend money on is really important, wait a month first.
As personal finance blog Be More With Less points out, waiting thirty days can mean the difference between desperately wanting something, and forgetting it ever existed. Sure, it may be on sale right now, but you're not saving money if you're buying something you don't need. If you really do want or need that item, it will probably still be there in a month:
Timing is everything, especially when it comes to an impulse purchase. Delay the purchase for 30 days and see if you are still as passionate about the purchase as you were initially. You may discover there is more joy in living without.
Of course, that's not to say that all expenditures ever should be delayed. Pay your rent on time. Get your car fixed when it breaks. However, for unplanned purchases or nice-to-haves, your choices will be the most biased in favour of spending in the heat of the moment. If you really want to save money, get in the habit of waiting to buy the thing you want.