Here's an easy one - when using a kettle, put as much water in as you expect to use, and turn the kettle off when it's at the right temperature.
Here's the maths: for every degree temperature raised, and for every 1ml of water, you need 4.3joules of energy. So raising one litre by eighty degrees takes 1000x80x4.3=344,000 joules.
To figure a cost for that, convert to kilowatt-hours:
1 watt == one joule per second
1 kwh == 1000 joules used continuously over a period of 3600 seconds, ie, 3,600,000 joules
At about 10p/kwh, that's roughly a penny, to raise a litre of water to just under boiling point from room temperature.
Trouble is, if you actually boil the water, you use a lot of energy just to turn the water into steam - and you don't pour steam into your tea so it's just wasted.