This tip applies when you’re flying Southwest Airlines or any other carrier that only offers open (unassigned) seating. If you’re among the last to board and all the good seats (i.e. aisle and window seats) are taken, look for couples who are traveling together but have snagged a window seat and an aisle seat, hoping that no one will want the middle seat between them.
When you take the seat between them, one of them will move so that they can sit next to each other and you’ve just scored yourself an aisle or window seat. Works like a charm