Things are cheaper when you wait to buy them
Many of us get FOMO when something is on sale and we buy it because we want the deal.
One of the ways I brainwash myself for the better is to constantly remind myself that things are cheaper when I wait to buy them.
The reason is that you spend less overall when you wait. So even if you spend more on that specific item later...if you spend within your monthly budget, you save far more money over the course of one year than if you bought all the good deals.
Whenever I see a good deal on something I want but I've already hit my shopping budget limit, I just remind myself of this fact, and it really does help.
It forces me to step back and look at the bigger picture of my spending rather than hyperfocus on one sale.