Don’t run the tap when brushing your teeth. Take showers rather than baths. Stop the shower when you’re soaping up or rubbing in shampoo. Put a brick in your toilet cistern to reduce water use.
You can also install a grey water diverter to channel waste water from your bathroom (so long as it’s above the ground floor!) to your garden. But if you do this, then make sure you don’t use toxic bathroom products.