There are 3 ways to do this:
You can keep using your favorite text editor and use an external Markdown preview program. We like the idea of this, since we are very partial to our text editors. If you use a Mac, you can use Marked, which is a standalone Markdown preview program.
You can use a WYSIWIG Markdown editor. See this article for more information.
Please note that none of these options includes any of the features unique to Leanpub-Flavored Markdown or Markua, like image size tags, for example. These options will only properly process "normal" Markdown.