if you want SUPER CHEAP and your flight is in over a week from today, put an ad on the craiglist
http://washingtondc.craigslist.org/ or in the citypaper - there are a ton of random people that do trips back and forth up the east coast looking for someone to share the cost of gas...
i hitched rides from harrisonburg, va, to dc and back before, but that's only a 2hr ride. depends on how much adventure you like in your travels.