A pair of comfortable shoes is your must-pack item. This is a walking town, and if you fail to pack for it, your feet will pay. D.C. isn't the most fashionable city in the country but people do look neat and presentable; business attire tends to be fairly conservative, and around college campuses and in hip neighborhoods like U Street Corridor or Adams Morgan, styles are more eclectic.
The most important element to consider, however, is the weather: D.C.'s temperatures can be extreme, and the right clothes are your best defense.
Winters are cold but sunny, with nighttime temperatures in the 20s and daytime highs in the 40s and 50s. Although the city doesn't normally get much snow, when it does, many streets won't be plowed for days, so if you're planning a visit for winter, bring a warm coat and hat and shoes that won't be ruined by snow and salt. Summers are muggy and very hot, with temperatures in the 80s and 90s and high humidity. Plan on cool, breathable fabrics, a hat for the sun, a sweater for overzealous air-conditioning, and an umbrella for daily thunderstorms. Fall and spring are less challenging, with temperatures in the 60s and occasional showers. Pants, lightweight sweaters, and light coats are appropriate.