Leaving the oven door open after use allows the excess heat to escape into the room, effectively warming the surrounding area.
When a soap bar is so tiny you crush it into the new soap bar while showering or bathing, adding some ‘extra’ to the new one.
Reuse ziploc bags, save rubber bands and tinfoil. Save bread bags. Use old T-shirts for cleaning rags. Use the back sides of used copy paper.