Twitter etiquette - careful with DMs

A load of people have asked me to write about this. DMs - Direct Messages - are a source of enormous irritation on Twitter and I feel the urge to lead you gently (or possibly even crabbitly) through the etiquette of this thorny subject. After all, lovely writers, you don't want to bug the pants off people and make them not want to read your book, do you? Or make them unfollow you? (Because that