BAD HABIT: Improper Gas Caps
Believe it or not, not clicking your gas cap completely tight could actually lead to evaporated gas over time. It's reported that improperly sealed gas caps actually evaporate hundreds of millions of gallons gasoline every year. To prevent this, make sure you hear at least 3 "clicks" when you're done filling up. This helps gas from evaporating and saves you money at the wallet.
And another tip: Always fill up the tank completely instead of opting for $10 or $20 partial fill-ups. The less trips you have to make to the gas station, the more fuel you save.