Avoid spending money on stuff you do not use.
Do an activity with indequate gear.
More than that, inadequate everything (wrong clothes, wrong food, wrong equipment) for a month before you buy special gear. You will either figure out that you do not need the gear, or you will have a better understanding of the characteristics of gear that you do want. You may also figure out that you do not want to do the damn activity anymore.
Borrow or rent things that you are only going to use once.
If you find yourself renting it 3-4 times, then consider it, but still do not make it a rule.
Avoid status items.
If you are going to buy a status item, buy one that you can, well resell. If you buy a car, for example, that is a status item, that might cost you $15k to $30k more than a non-status item car. With $7k you can buy a nice watch and sell it for $5k if you realize that you do not need it. Less money tied up and more money that can be recovered. Still a $2k lesson.
If you are going to buy, buy on sale or used. A nice $5k watch may sell for $5k plus maintenance costs later. However, you need knowledge or hire expertise to make a good purchase like this.
If you are going to buy a status item, go ahead and buy a status item. Buy the thing, not a thing that is kind of like the thing.
Go shopping in your house.
If it is stuff you want, you might want to check what you have in your house or apartment.
Find your meditation. Lots of times I want to buy things because I am stressed out, or someone insulted me, or I am feeling lonely or sad, or I have not done work and I want to do something. Meditation of whatever sort helps you clear that out.