Churning credit card and bank account sign up bonuses. Bank account bonuses unfortunately taxable, but credit card bonuses aren't.
Doesn't that hurt your credit score?
This isn't really a "hack," but the best thing that I ever did was put any large chunks of cash that I received (bonuses, gifts, etc.) into an S&P 500 index fund, starting in my early 20s. I'm now in my late 40s, and that index fund has done incredibly well for itself over the years, with relatively modest contributions.
Living in the city and not owning a car has been a win for me as well. Yes, the rent is high, but I have a good quality of life and don't have to worry about maintenance, parking tickets, and all the rest. I rent a car about once a month, and that costs far less than ownership of even a modest vehicle.
I don't generally do auto-pay on anything where the amount of the bill can vary from month to month. I'm paranoid about billing errors (which happen from time to time), and can say from experience that it is easier to fix them when the bill has not already been paid. I don't use debit cards for the reasons given above (no liability protections, etc.), and generally use cash when buying small (< $200-ish) items in person. Credit cards (paid in full, every month) are good for larger items, as they provide a record of the purchase (good for warranty purposes) and also usually offer some protections against defective products or misbehavior from merchants.
I also call up every provider of a subscription service that I use (telephone, Internet, etc.) on at least an annual basis and ask for a lower price. It works often enough to justify the time spent doing it.
I generally shop for refurbished or open-box items when possible. I don't normally do extended warranties, but I am a believer in them for laptop computers (not desktops, though) and washing machines.