Compare seats. Flying in coach means different things on different airlines—carriers offer anywhere from 30 to 42 inches of legroom. Planes with two aisles, such as a Boeing 767, generally provide “bigger seats, more legroom, and larger overhead bins” than single-aisle aircrafts. Check SeatGuru.com to compare different airlines’ seats.
Time it right. Flights booked for certain times are more likely to have empty seats, which makes switching to more comfortable seats easier. Midweek flights in the afternoon are usually emptier, although that’s “not the case for all destinations.”
Keep checking. As the flight date approaches, “return to the airline’s site several times” to see if better seats have opened up. Then check in online before heading to the airport; that’s when new seats most often become available.
Source: Condé Nast Traveler