Let your hot water cool before you pour it down the drain.
Hot water holds a lot of energy. Don't pour it down the drain while still hot, but let it cool off (i.e. heat the room) to room temperature first. Pick up the potatoes from the pot. Or pour the hot water on top of your dirty dishes. Or put the plug in the sink for a while. Let the bath cool off before you let it out.
One litre of boiling water gives 0.2kWh of free heating. [http://www.gronatips.se/index.php?id=11]",