I don't know about a preview button, but I wrote a mammouth tome about DBI in the Code Catacombs last week.
I dutifully installed a <READMORE> after the first
few exciting paragraphs designed to capture your attention,
and then blabbered away for hours.
Imagine my shock and horror when upon submitting I
see that the whole post appears in all its gory details.
I can still see in the source the place where the
<READMORE> appeared, was recognised and dutifully
stripped out, but there's no "Read more" link on the page. So shoot me.