My personal all time favorite tip is
STOP CARING WHAT PEOPLE THINK OF YOU!
Buy the things you need, not the things you want. Buy clothes that are functional and high enough quality to function well, but untrain that need for image, brand, etc. Fewer toys, but ones you really use, are more valuable.
I got to perfect this one most as a poor student, then poor grad student, then poor post doc. I own clothes, furniture, and other 'stuff' from 5-10 years ago that are perfectly functional. When I actually do need something, I just buy it and don't waste tons of time on coupons. I have no credit card debt, car is paid off completely, and I just increased my salary 2.5x without changing my standard of living (finally out of academia and into a 'real job'). I'm going to use all this extra money to save 100K in 4 years for a house because I don't need to reward myself with unnecessary little stuff along the way. I think that's the most valuable mental shift in the world.