Always think secondhand first for everything. Try to get things for free first on Freecycle/Facebook free groups/Olio. If you use these also give away items you don't need, you'd be surprised what you think is rubbish but people will queue up to collect for free, it's much more convenient for the IP not to mention obviously so much better for the planet and local community.
For clothes and shoes look at Vinted or eBay. For home/furniture items eBay end of line/seconds and Facebook marketplace. Toys and children's clothes from car boot sales. Charity shops are an option too although I find them a lot more expensive and much lower quality than the above options.
Don't upgrade your phone and switch to a SIM only plan, Money saving expert has all the best ones starting at about £3 a month.
Compare every year for things like insurances to make sure you always get the best deal (I set yearly reminders in my phone for this). Pick a comparison site with the best perk. We use compare the market because the saving on cinema tickets pays for our home insurance for example but the others offer other perks that might be better for you.
Break up with TV subscription sites when you've watched everything you're interested in and then go back when they send you a good deal. I've got two months of Now TV for £1 a month at the moment.
Use Olio (food sharing app) or a community fridge for food (they distribute surplus food for a small donation, not a food bank, open to all). Use everything, throw nothing away.
Sell anything you can that you're not using any more.
Give yourself some time before buying anything non essential to see if the desire fades, it often does.
Think about your priorities and switch/downshift/change to secondhand/reduce the ones that make the most saving for minimum cost to your quality of life. So for example I don't mind at all getting all my clothes and shoes etc secondhand, this literally increases my quality of life for no cost so I do that. But I use my oven every day instead of doing one pot cooking/using as slow cooker because I've decided that's worth it for me. I don't much like baths or long showers so I have a 2 minute shower every day. I like going to the pub most weeks but controversially I like Spoons so we mostly go there which saves minimum 60% on drinks prices. We don't like eating out much so don't bother with it, we prefer a takeaway or delivery so have one of those occasionally instead.