Rain gardens are ideal for DC because they mimic the region's pre-settlement swamps. Rain gardens are designed to slow rain water and trap it in a way that's good for your garden and the environment. The Washington Post features local rain gardens and describes how local residents have built their own. More info below the jump...
The Post article details how experts have built their own rain gardens by digging up to three feet or more, and using surplus clay to reform the soil (varying combinations of soil, clay, compost and mulch recommended by different experts).
If you plan to dig deep, make sure you know the locations of buried utility pipes and wires.
Also choose plants that are suited for the depth of your changed soil. According to the Post, the lower the depth of your altered soil, the wetter the soil, and so the greater the need to select flood-tolerant plants and flowers.
Have you built a rain garden?
[photo above the jump from the Washington Post]