This one’s pretty stressful, but I find it effective.
Let’s say $X is your total (inflation-adjusted to the present day) lifetime earnings from the path you’re on in life.
You’re only working with $X, and your goal is to be as happy as possible and/or do the most good with that amount of money.
Think of every purchase you make as coming out of this budget.
When you have a job, it’s easy to feel like the money will always keep coming. However, there’s only so many years you can work and only so many years you’ll want to work.
Obviously there are limitations to this technique, since some purchases may increase your future earnings. However, most purchases don’t, so I think it’s important to be aware of how efficient a given purchase is at turning money into happiness relative to other uses of that money.