Nobody likes to feel squished, especially on a cross-country flight.
Next time you and your travel partner book your flights, reserve the window and aisle seats.
This makes it less likely that another passenger will reserve the middle seat.
If it happens to be a full flight and someone does sit in the middle seat, simply ask if he or she is willing to swap seats with one of you so you can sit together. Chances are, the passenger will not only honor your request, but be grateful to not be sandwiched between two strangers.