First let's figure out your toilet's water use. It should say somewhere on it how many gallons it uses per flush. If it uses 1.6 gallons per flush, it is meeting the federal standard. Bonus points if it says 1.28 gallons per flush and meets the Environmental Protection Agency's (EPA) WaterSense standards. It'll save 20% over the 1.6-gallon model. If your toilet is an older, much less efficient model it is likely using 6 gallons of water per flush and costing you an extra $100 per year. According to EPA, if all of the inefficient toilets in the country were replaced with WaterSense models, it would equal 640 billion gallons of water savings per year. That's the amount of water that flows over Niagara Falls in two weeks.
Whether you already have a low-flow toilet or not, be sure it is functioning properly and not leaking. To do this, open the back of the toilet and drop in a few drops of food coloring. If that color appears in the toilet bowl within the next 15 minutes or so, you have a leak that needs to be fixed as soon as possible. (Tip: Do this test right before you are going to flush it anyway to avoid staining.)