Search online real estate listings. Start with sites that allow you to fine-tune your search according to price, size, and geographic location. Also look at Washington-specific sites (see Resources).
Peruse the classified ads in the Washington Post and the Washington Times, the two papers with the most circulation. Weekend editions tend to have more listings.
Check the data on recent home sales in Washington D.C. The Washington Post's home buyer's guide (see Resources) is a valuable source of information. Search for sales and tax data on home purchases in the metropolitan area and use it to get a sense of the cost and dynamics of the market. You can even narrow the search to the specific street or seller name.
Drive through neighborhoods that appeal to you. This may not be the most efficient means of research, but it allows you to get a feel for neighborhoods you may be interested in, and come across houses for sale that may not be listed in other sources.