I either remember where I was anyway, or use tickets and boarding cards. But mostly I use postcards.

The one I've had the longest - it must be getting on for 14 years - is a strange one that was posted to me by someone who clearly didn't know my full name, and had to find out my address (it's in a different coloured ink to my name). It has a mysterious, possibly insulting, message scrawled on the back, which is scarcely legible, and it's unsigned.