Also keep in mind that converting directly from DOCX to EPUB will work much better than going through the filtered HTML intermediary. Kovid wrote a very nice DOCX plugin, why not use it?
Alternatively, use Toxaris' Ebook-tools word addin, which will export to EPUB.
But yeah, proper styling comes first.
Those two together will go a great deal towards a better-looking book.
For the best results, manual tweaking using calibre's ebook editor is a wise move.
Knowledge of HTML helps.