(1) If you want to hop onto an earlier flight, make sure your luggage gets transferred to the new flight at least 2 hours prior to take off. Or if you come from abroad, make the change right after customs, right before you enter your luggage back into the hands of the airline.
(2) When Terminal C (an older United Terminal) at Dulles is stuffy and without air-conditioning (as was today), head over to Terminal B. Terminal B is a new international terminal. It was not very crowded and had some nice food options. At Dulles, it's only a 1 minute ride on those odd looking "mobile lounges". Unlike other airports where you might fear coming back via the inter-terminal transportation system can take a long time, the system at Dulles is great and fast.
From the history page of Metropolitan Washington Airport Authority page:
The concept that made the new airport outstanding and unique from the passenger's view in 1962 was the specially designed mobile lounge, used to transport the passengers between the terminal building and the aircraft parked on a jet ramp ½ mile away from the terminal. The mobile lounge was designed by the Chrysler Corporation in association with the Budd Company.
The mobile lounge was constructed as a 54-foot long, 16-foot wide, 17 1/2-foot high vehicle, and could carry 102 passengers, 71 of them seated, directly from the terminal to the aircraft on the ramp. This protected the passengers from weather, jet noise and blast, and also eliminated long walking distances. Because of the mobile lounge, passengers had to walk only 200 feet once they entered the terminal until they were seated in the lounge for the short trip directly to their aircraft.
Today, Dulles operates 19 mobile lounges and 30 plane mates, which are similar to the lounges but can transport passengers from the terminals, directly onto the airplane by attaching itself to the aircraft.