Your fuel pump sits inside the gas tank, and the fuel around it helps keep it cool. When you often drive with the tank almost empty, the pump can run hotter and wear out sooner. Replacing a fuel pump often costs several hundred dollars, sometimes more than $1,000.

A low tank can also pull dirt from the bottom of the tank toward your fuel filter. That's why this habit goes hand in hand with a second one: change your fuel filter on schedule. Many carmakers recommend every 30,000 to 50,000 miles, but some newer cars have a filter inside the tank that isn't meant to be changed often. Ask your mechanic which type you have.

Most drivers never think about brake fluid. But here's what many people don't know: brake fluid slowly absorbs water from the air. Over time, that water lowers the fluid's boiling point.

Why does that matter? On a long downhill, like driving down I-70 from the mountains, your brakes get very hot. If old fluid boils, it can form gas bubbles, and your brake pedal may feel soft or sink toward the floor. That's the moment you least want it to happen.

Water in the fluid can also cause rust inside expensive parts like the brake calipers and master cylinder. A brake fluid change is a small job that keeps your pedal firm and your braking strong. Check your owner's manual for the right type of fluid and the right schedule.

Cheap or worn-out spark plugs can cause a misfire. That means the fuel in a cylinder doesn't burn the way it should. Unburned fuel can then go into the exhaust and overheat the catalytic converter, which is one of the most expensive parts on a car.

So a set of spark plugs that costs less than $100 can protect a part that often costs $1,000 or more. Use the type your engine calls for (some need iridium or platinum plugs), and replace them on the schedule in your owner's manual.