Only fill the kettle with as much water as you need.
Another one that I already do. So let's find something similar that I can do instead. Save Cash and Save the Planet suggests various ways of using less hot water, of which one is to put the plug in when you wash your hands, so the water's not pouring down the sink. I shall do this from now on, and also I will turn the tap off when I'm cleaning my teeth rather than letting it keep running.