There's no sense paying to water your lawn when we get the water free on a regular basis!
There is already a post in this forum on using a rain barrel which is great for smaller applications. If your needs exceed that, I posted a comprehensive thread on how to build a Rain Harvest Water System to store hundreds / thousands of gallons for whatever usage you need. I actually use mine to supply all the fresh water to my house and the design is described in detail, and to apply that to a landscaping watering use is a much simpler subset of that.
Here is the URL:
How to Build a Rain Harvest Water System And if that gets stripped (in some forums they do) its in the 'Going Green' section, 'Solar & Wind Power, Geo-Thermal Energy Systems and Water Conservation' sub section.
Enjoy!