Download this free Attachment Scanner plugin for Mail.app, and install it following the directions on the developer's site. Now when you type "attach," "attachment,", "attaching," or similar words in an email, if you hit Send without actually attaching a document, you'll see a warning like this one: "Message Has No Attachment. Your mail appears to refer to an attachment, but none exists. Do you wish to continue?" If you don't mention an attachment, you won't see anything different than usual. I've been using it all day, and really appreciate getting a second chance to save face thanks to developer James Eagan.