Tip #1: "Wishlist"

Everyone gets an idea or already the urge to buy something a few times a week. Like for new clothing, a game, some device, etc. or even specific food/drinks. In that moment, write it down on a note or little list. Don't go to buy immediately, instead keep the note in some open place around. When you read it from time to time, then question yourself. "Do I need it now/soon? Why do I need it? Where would I keep it? Can I pay it immediately?" Give yourself enough time to answer all possible questions.
If you are still certain, you want this item, then finally buy it. Or leave it, if that item became uninteresting to you.

Examples: You read about a game you find exciting. Put it on your [generic game store] wish list. Check it out later. Look for the pricing and possible offers. Wait a few months until you got the budget and time to play.
You see some shiny new dress or accessoires in a magazine. Take a note, with the price(!) and put it on your note board. Another day, check the product, the shop and where it was made. Wait. Think about it. Check your wardrobe. Still need it?

TL;DR: Put stuff you want on a wish list. (Both for big and for small items.) Don't buy directly but check the list regularly. Slow it down, think it through. If after some time and good questioning you still want it, then go.