I obviously don't send postcards often enough. I guess that's why I can never seem to remember the simple rule: the stamp goes on BEFORE the message is written. Stamp before you write. Quite straightforward.
Clearly the way NOT to do it is to write up to the edges of that tiny rectangle marked 'affix stamp here', start to get a funny feeling as I am finishing up my message and signing off, realise before I even get a stamp out that it's not going to fit, and then have to decide whether it can go on with a little bit of overhang or whether some words have to be rewritten in the margins. Neither is a great option: the former because, well, you never know when the post office pixies might take offence and redirect it to the Dead Letter Office; the latter because it is always noticable that the stamp has covered some words, and even if you rewrite them elsewhere, the recipient doesn't know that and might still think they have missed out on something.