Rent for longer
If the rental car company doesn't charge a fee for early returns, it can sometimes be cheaper to rent for longer (to get, say, a weekend or discount rate) but drop off before your rental period is up.
Ask for a grace period before you drive away
Yes, car rental agencies play hardball with their 24-hour rule but most do have a grace period, often up to two hours -- you just have to ask beforehand. They'll often give you more time at no extra cost.
Refill the gas tank with the car on
Sure, you're technically breaking the law, and it's not that safe, but if you leave the car running while you slowly refill the tank, you'll be able to more accurately nail the level of gas you drove off with -- thus ensuring you don't hand the rental company extra money by overfilling.