Although it's nigh on impossible to visit the flight deck like the good old days, they often open the door when the aircraft has parked. On the rare occasions when I have flown I have taken the opportunity to poke my head in the door and say "I'm a retired Heathrow controller; thanks for the flight". I have always received a very warm welcome... even a friendly "telling off" from an ANZ Captain for not making my presence known before the flight!