When you clean your fish tank, use the water you've drained on your plants. The water is rich in nitrogen and phosphorus, providing you with a free and effective fertilizer.
When mopping, use a mild phosphate free detergent. When you're done cleaning, dilute the mop water and then empty onto plants outside the house. The key here is to use environmentally safe products!
When you give your pet fresh water, don't throw the old water down the drain. Use it to water your trees or shrubs.
Bathe your pets outdoors in an area in need of water.
Drop that tissue in the trash instead of flushing it and save gallons every time.