If you're real ambitious, you can set your home up with a 'grey' water system. This will feed your toilet tanks with shower, bath, waste RODI, dishwater depending on how elaborate you want to go. I think all you need is some sort of pressurized holding tank to collect the water and either re-plumb your toilet tanks to it or have a valve so you can use either grey or fresh.
I use the waste in the garden in summer and lug buckets to the toilet in winter. Water is a little more precious here in the sunny Okanagan, or so the city/region officials will have us believe, since they keep cranking up the rates every year and put us on watering restrictions during the summer.
http://www.bracsystems.com/
http://www.thenaturalhome.com/greywater.html
http://www.gizmag.com/go/6810/