Step 1: Required Materials
One (1) half-gallon plastic jug (one of the small, tall ones)
Step 2: Preparation
(Wow, mine's kind of rusty. I should probably clean it...)
Step 3: Finale!
Now, once most of the water is gone, put your water-filled jug in, in the corner, where it won't mess with the flushing parts.
Watch it fill up. The jug is now displacing some water, and as the tank fills, it reaches the float sooner, which stops the flow of water. Every time you flush, you will be using 1/2 a gallon less water. If you have room, you can even put in two or three of these, but then it may affect the amount of water in the bowl, which may not be good. Fiddle around with it!