When I was doing something like this I used a 'no card purchases' rule, meaning if I bought something it had to be paid for from my walking around money that was also used for the odd cup of tea or scone while I was out, or lip balm, or pack of tissues, etc. I was strict about my cash and didn't just go and get more. I had a weekly limit. I still bought stuff, but then had to live with the decision until the following Monday morning.
I also stopped looking at magazines and stopped buying them outright, so I wouldn't feel I was missing something or untrendy.
When I was losing weight I bought clothes in second hand shops and that is now where I do most of my shopping. I recycle items I haven't worn for a while, those that didn't work out/were too fussy for laundry purposes, and those that get too baggy, etc. It's a bit like a clothing lending library for me.
I also went through a period when I had no money to spare for anything, and missed my old habit of browsing and occasionally buying something. So I would go for a walk or listen to some of my favourite music. I took up embroidery again -- it cost very little to start and I could work on something whenever I felt like it. It made me realise shopping was just a habit that I could change.
DD1 has a photo on her phone of every single article of clothing she owns, and if she is tempted by something when she's out she whips out her phone to see if she already has a similar item, which she normally does. It also shows her how many clothes she really has. DD2 keeps a photo of her bedroom closet on her phone, with the door open and the closet in all its bulging overflowyness telling her 'just say noooooo'.