A how-to on WikiHow has been making the rounds on some blogs and I figure I’ll add to that here since many places in the US are still under moderate to severe drought including my part of the country. The article is “How to Convert Any Toilet to a Low Flush Toilet” and the just of it is to take a half-gallon container of milk or juice, remove any labels, put in something to make it sink (pebbles, sand, gravel, whatever…), and then add water so it doesn’t move around. Take this unit you just put together seal it shut (put on the lid) and place it the tank of the toilet so it doesn’t interfere with the mechanism. Replace the lid and then use as normal. The container will displace an equal amount of water up to a half gallon depending on the type of toilet you already have.
There are a number of other things you can do if you feel to lazy to put together this milk jug, simply putting a rock in the tank can have the same affect, of course you’ll want to be VERY careful not to crack the tank and make sure to clean it off very well before you put it in the tank. As you can see you can use about anything to take up space in the tank, the big criteria is that it doesn’t get in the way of the mechanism or damage the toilet or septic system.