If you are unable to afford purchasing a new low-flow toilet or those dual-flush kind that use different amount of water depending upon what type of waste you are flushing, then consider putting a brick or a plastic bottle filled with sand in the tank of your toilet.
The average toilet will use 3-6 gallons of water per flush. It really all depends upon how much water fills in your tank. If space in your tank is filled with something else (like a brick) that won't flush, then less water is used. It is estimated that 30% of a household's water usage comes from flushing. So, it would reason a good way to reduce your water usage would be to cut back on how much is used with flushing. You really only need half as much water to flush efficiently.