Some really good advice from @suki1964 on p2, especially about knowing the price of everything and expanding where you shop. So many people pay way over the odds out of habit, misplaced faith, or sheer thoughtlessness. And, if you know prices, when you spot a bargain you stock up.
I'd also channel my inner Warren Buffet and add trying to distinguish between the price of something, which is what you pay, from the value of something, which is what you get. This is not as simplistic as 'you get what you pay for'.
A couple of years ago we were in the market for a new car. We had no particular budget, just some fairly fixed requirements, and the final shortlist came down to two: a Dacia Jogger at £24k and a Skoda Kodiaq at £40k. The Skoda was clearly the nicer car, and if we had thought that the extra £16k gave us that much value we would have happily bought it, but we just couldn't see it.
Similarly, if you travel around Europe enough and realise that you can get a really good cup of coffee served at a café table for €1.20, then you become quite reluctant to queue up in a chain and pay £4.50 for something rather less tasty which you have to carry to a (filthy) table yourself - if you can find one.