P.S. for "post script"?
Q. (from Mary) I always used P.S. for "post script" until someone who was an English major told me most people get it wrong...that it really should be stated PS: Which is correct? Do you know why he said it should be PS: ?

Thank you,
Mary

A. Mary, as far as I know, the trend is to write PS as capital letters without periods. As for the colon, my guess is that it is written PS: by analogy with abbreviations such as TO: that are used in memoranda and formal correspondence.

For more:
http://www.udel.edu/eli/g01.html