I have little doubt that I spend more than I ought to, but one of the things that has really made the difference between "not the greatest fiscal discipline" and "individual human equivalent of Greece" is that I learned long ago not to buy things I'm just not going to use.
I mean, I'll buy junk, but it's all things I'm actually going to do. Like when folks tell me to grab something on Steam because it's cheap, I don't. Even though it's a great value, I know I'm probably never going to get around to play whatever the game of the day is, so I just don't buy it. That's why I never have any posts in the "games I gotta get around to playing" thread.
I try to extend that to everything, just not buying things I'm not going to use. I mean, that seems obvious, but in an age where everything is so heavily advertised and where microtransactions and sales where the price is lowered enough to make not buying seem "foolish" (Steam sales and the like), impulse buying has gotten easier and easier. I mean I know people who now spend like $75 a month on $1 apps. There's no WAY they're using even a tenth of those. This is also the biggest reason I don't own a car. Of course I've lost potential jobs over not owning a car, but never one I was upset to pass on.
As a result, probably my biggest unwise regular expenditure is food (it's too say to rationalize "eating well") and maintenance costs. I'm at an age where not a month goes by that I don't have to replace something or buy something for the apartment (parts for one bike or another, furniture or the like, clothes, etc.). Even when it's way overdue (my motorbike helmet was so banged up and worn out that last year the instructor refused to let me use it), I still feel like "hey I could have used this old item just a bit longer...". Sometimes (like the motorbike helmet), that's silly thinking, but sometimes it's not.
The real problem is that my income is adequate to survive, but is in no way adequate to improve my lot. Even if I lived like a monk and trashed all discretionary spending, I would save a few grand per year at most. A few grand is certainly nothing to sneeze at, but if I want to save for school, start a business, or do ANYTHING meaningful, I would need to be making at least twenty-five percent more than I do now.