In the great supercity of Auckland, NZ we now have to pay not only for the water that is reticulated into our property, but also for reticulating it out (the "wastewater charge" is based on how much comes in rather than how much goes out through the wastewater system, so even if you use most of your water on your garden, you still pay a wastewater fee). There is debate about the justice of this, but here's a way we're reducing costs by reducing the water coming in*.
Watering potted plants is more efficient if they have trays under them, but the thirsty ones run out faster than the others. Putting them all in the same "tray" means they can share.
In our case, this project also helps hide the untidiness under the deck...
*We're also using our shower and bath water to irrigate a "compost biomass" garden (comfrey, sugarcane, culinary ginger, etc).
Step 1: Materials and tools
polythene (the thicker the better)
duct tape (gaffer, duck, etc)
hose fittings (see picture)
drill (preferably electric)
craft knife (although a decent pair of scissors would be better)