I picked it up from Salinger. I find that if I can't eradicate parenthesis by some other means, or if it's more effort to do so than I want to spend, em-dashes usually replace them without doing any harm and aren't quite so ugly, aside from being useful in other cases. In particular, parenthesis at the end of a sentence are awful, while a single em-dash does a similar job much more neatly and looks totally natural.