When you start thinking in terms of ROI on happiness, some choices become pretty natural. Even more so if you convert dollars to time spent sitting in your chair at work, even more so when it's compounded at 4-8% yearly.
In other words, at some point I started to think how much time I'll have to spend at work for a nice car, thus I drive a 3000$ used Kia that's in good shape with low mileage. Am I any less happy? Maybe 0.5% less than if I had a cooler car, but I'll be spending probably 2-3 years less at work so that's good ROI.
Start doing that for a bit and you've got most of the work done. That doesn't mean being a cheapskate. I don't care how much my daughter's gymnastics class costs, that's good ROI. Having a nice logo on my shirt gives me jack squat so no money there. I'm trying to push off on the whole family Disney trip because I'm sure it's great memories but I'm pretty sure we can juice more happiness out of the little fortune that would cost.
Also, not just taking for granted that you have to pay for stuff because that's how it is. The 3000$ car is only insured for liability. If I crash it and I'm still alive I'll buy another one cash.
Living below your means is freedom. My girlfriend was able to not work a whole year to stay with our second born because we lived off one salary. That cost me a few 10Ks out of pocket. No problem. I'm penny pinching where it doesn't matter so I can spend where it does.
I have a number in mind I can probably reach within 4 years (I'm 40), then I just won't have to worry about the money coming in, but I'll still want to work at projects, jobs (though not full time), independently of how much money they bring in (though I'd like it to be >0).
I already had pretty much that mindset but Mr Money Mustache accelerated it.