This is a great pdf. The state of Oregon has a whole assortment of info on harvesting rain water. From downspouts and filtering to underground cisterns. If it is legal to collect rain water in your area I think most of these ideas would work.
Rain Water Harvesting
Fiskars 5962 Rain Barrel DiverterPro Kit, Black
The Complete Guide to Water Storage: How to Use Gray Water and Rainwater Systems, Rain Barrels, Tanks, and Other Water Storage Techniques for Household and Emergency Use (Back to Basics Conserving)