If you are not ready to install a green roof yet, you can accomplish much the same thing by directing water from your roof into a rain barrel. It's actualy as easy as it sounds. You get a barrel and you put it under your drain pipe on your home or brownstone. If you are a renter, you could always keep one on your fire-escape when it rains and bring it in after the shower to use on your plants.
What is a rain barrel? Rain barrels are low-tech devices that collect rainwater for home or garden use. They are an inexpensive way to save and reuse water and cut down on your monthly water bills.
Here are some sites that provide detailed instructions on how to build a rain barrel:
The main goal is to reduce the amount of storm water that runs off your property and into an overburdened sewer system. And if you save that runoff and use it to water your plants, lawn or garden, you will be both conserving a valuable resource and saving money on your utility bill.