It's easy! Just slap an ol' gallon jug in the tank.
Step 1: Get Gallon Jug
Any one will do, then empty it out. If it contains milk you are required to drink the gallon in one hour or this trick won't work.
Step 2: Fill Jug With Water
If you really want to conserve more water, you can pee into the jug. I mean, it's going in the toilet anyway...
Step 3: Find Toilet, Cram in Jug
Open the top of your tank and slap the jug in there, the water(or pee) in the jug will keep it from floating. It might take some persuasion to get the jug in there, just make sure it's not blocking any important mechanisms.
Step 4: Enjoy Saving the World While You Take a Dump!
A word of warning, if you happen to pack a mean bowl the lack of water pressure might become a problem. Although I have never encountered this problem, it is in my understanding that some of you heavy hitters might. Good luck.