I send postcards, and I echo what others have said about people being grateful - I usually pick out special ones - skyscrapers for a friend who likes those, castles for another friend who is really into history and haunted places, that sort of thing. I hate writing them out, because I never know what to say, but they still appreciate it even if I just say 'Hi, got into XYZ today, really tired, but saw this and thought you would like it...'

I think it means a lot to people to know you took the time out of a trip to think of them.