Don't fly on a Monday, Wednesday or Friday. Most business travel is during those days and it accounts for the high volume. If flying during the summer months always book a 5am or 6am departure as nobody wants to travel at that time. When picking an airport to change at especially when flying Delta, US Airways/American or United, heck any of the airlines avoid a plane change in the "northeast corridor" that includes airports such as EWR (Newerk), JFK (Kennedy), LGA (New York LaGuardia), BOS (Boston Logan), PHL (Philadelphia), BWI (Baltimore), DCA (Reagan National) or IAD (Washington Dulles) most of the flights in or out of these places are delayed significantly due to the heavy regional jet traffic and what is called "stacking" stacking is where planes are put into holding patterns and in the PHL, EWR, JFK and LGA airspace, they all intersect each other and cause planes that are in holding patterns for PHL to be backed up all the way out to Harrisburg, when it starts to encroach south it hampers airspace for BWI, DCA and IAD. DCA has the most restrictive airspace out if any airport in the world because if it's close proximation to the capital, runway 33 for example at DCA is directly inline with the White House which makes landing a very difficult challenge and the River Visual LDA approach to Runway 19 is one of the top 3 hardest approaches in the United States. So with all the small 50 seat regional jet traffic jamming things up it makes it much harder for international and coast to coast "mainline" flights to depart on time.