There's been a lot of buzz lately about the alternative transportation options between NY and DC. Even the Wall Street Journal has caught on to the noise. Rob Goodspeed from DCist posts on the subject.
Flying between DCA and LGA is no longer the best option. With the hassle of TSA staff at airports and the pain of driving in to NYC from LGA, the flight is not that appealing relative to Amtrak train services. The train is generally on schedule and, unlike driving, doesn't face delays due to weather or traffic. Plus, it's really convenient to get work done (with 3 hours of uninterrupted desk-time). But, it's usually the priciest option.
I've never taken a bus for the trip, but am definitely going to try in the next weeks. I've heard the Washington Deluxe is the best bet, with somewhat more upscale services to the Chinatown buses. Apparently, the movies played are actually in English rather than Mandarin!