Deactivate automatic mark as read in Apple Mail 7.3
I've been using Mail Act-On. It's overkill (does a lot more than just keep messages unread) and kind of pricey at $30 (with free trial), but since it's a paid app, they keep it up to date. Even when TruePreview was being maintained, it'd sometimes be weeks or months before it was updated to work with the latest version of OS X.