This is a suggestion of a feature I love from Gmail. An Undo Send button for PMs.
I'm the kind of person who writes a post, previews it several times and makes sure everything's perfect, then as soon as I've hit "post" realise I've forgotten or would like to clarify something. I've found Gmail wonderful for email because of this, you can enable "Undo Send" which delays sending your email for about 30 seconds. In this time you can click Undo, and go back and edit it, or even not send it at all if for some reason you've changed your mind.
I'm not sure if this is possible in the slightest for PMs, but if it was that would be absolutely great.
e.g. Within 20 seconds of me posting this I edited it for clarity. Twice.