I have a line in my budget called “disposable” which is a fixed amount per month that I’m perfectly happy to spend on me. It covers entertainments, videogames, books, anything that isn’t strictly necessary, but is enjoyable.
I keep this largely the same every year, not even adjusting it for pay rises etc. It’s a non-negotiable amount (unless things get really dire!) and helps me to understand what I have available to “waste”. Keeping it at a static amount helps to divert any extra into savings and investments, and helps me to avoid lifestyle creep.