Those small drips can add up to hundreds or thousands of gallons in a short time.
Place a few drops of food coloring in the tank of the toilet and wait 15 minutes (don't flush!) If the color seeps into the bowl, the toilet flapper is likely worn and leaking. Place the old flapper in a plastic bag and take it to the hardware store to purchase a replacement.
It's a simple step, but over time the savings can really add up.