Not having a car (always living/renting in walking or biking distance of my work)
Moving in with my partner straight out of college so we could split expenses
Moving (with partner) to a low-cost-of-living city for the first 5 years after college
Putting most of my medium-term and long-term savings into low-expense-ratio, passively managed index funds starting in my early 20s
Buying almost exclusively second-hand clothes, furniture, and cookery
Borrowing all desired books (and many desired movies and TV shows) from the library
Only buying games when they are bundled or otherwise on steep discount years after release
Pirating any other media in which I'm interested if its distributors make it even remotely difficult for me to buy it at a reasonable price
Planning all dinners in advance every week before grocery shopping (leads to almost never eating at restaurants or ordering takeout, and almost no food waste because grocery list is based on actual meals)
Now, if I had to choose the best financial move out of that list? Probably the index funds. Though not having to pay for a car (or car insurance, or car registration, or car repair and maintenance, or parking, or fuel) is a close second.