Keeping your receipts makes it easier for you to record your spending. When you write down the amount of money you spent in red, your brain automatically recognizes that number as a “danger” or “beware” sign, which may subconsciously help you notice how much money you have been spending. In my case, I write my spending on my calendar on the day I made a purchase. When I look back at a particularly extravagant month, I cringe from seeing all the red and the next month I tend to be particularly frugal to make up for it.