Put a brick in the tank of each toilet. It won't interfere with the flushing process, but allows you to save about a quart of water each time you flush.
Change the brick often, if they were poorly made they will start falling apart in the water and could cause some plumbing problems. Also, if you wash dishes by hand, use that water to pour into the toilet bowl to flush the toilet. Pretty much any water used in the kitchen can be used for that....water from rinsing or cooking vegies, washing hands etc. You can also have a bucket in the shower and catch some of that water to use for the same purpose.
Approx three to four gallons of water is used each time you pull the flush. To prevent water wastage, you can use dual toilet flushes or reuse gray water in the toilet. moderator's note: removed website link, see point 1.1 of usage rules
