Keep receipts and go through them, but the day after.
Make a point of noting what you bought and when you bought it. I got this tip from my sister who was spending a lot on her way home from work when she nipped in to the supermarket for stuff for dinner (her and her husband used to shop on their way home rather than do a big shop). And she would spend money on meal stuff but because she was hungry, about the same on crap that she fancied. They would eat the meal but the cupboards were bulging with crap that they werent eating. He was doing the same, so they made time to do a big shop once a week. They included a "crap" budget in that, and so they always had snacks if they wanted them but were spending far less.
She always bought a naice coffee on her way to work. Except the coffee wasnt that naice, cost a fortune but was nicer than instant. So she bought a coffee subscription that came with a free machine (Nespresso, sadly they dont do the free ones anymore but they do heavily discount decent machines with a sub). She was spending less than half what she previously spent and the coffee was nicer! She got that tip from me!
Same with clothes, hobby stuff etc. When looked at dispassionately she could see a pattern of treating herself after a bad day, him buying model stuff cos he was bored...that kind of thing. Once they realised what their pointless spending triggers were, they could see them off.
I started doing it and it was eye opening. I now always eat before going food shopping. I have instuted the 48 hour rule on clothes shopping. I wait 48 hours and if I still want it and I still think it is worth the money, I will buy it. I would guess that about 80% of the time, I dont buy whatever it is that took my eye.
Its not big money that fucks you up, its the "oh its only a couple of quid...." that does it because you dont even notice you are spending it until you realise that you had £20 yesterday and now you dont but you dont actually have anything to show for it.