Toilet flushing is responsible for about 30 percent of all water used by the average American household, which means that almost 44,000 gallons of quality water is flushed away per family every year.

Upgrading to a high-efficiency toilet would be ideal, but since you & I live in the real world, there are some simple things you can do to improve your current model.
I've used something called a tank bag to reduce the amount of water my toilet uses per flush. It's basically a plastic bag that you fill with water and hook to the side of the tank. You could also submerge a filled water bottle - but add some rocks or sand for extra weight, and place them carefully!

Don't use a brick - this will degrade over time and can harm the toilet.

A leaky toilet can waste up to 200 gallons per day! To see if your toilet is leaking, drop some food coloring in the tank & wait 15 min. If the color shows up in the bowl, you've got a leak. There are some things you can do to fix it, but you might need to call a plumber.