For my first dance with .mobi and .epub I used a mix of tools like Mobipocket Creator to export my .epub format to .mobi for the Kindle. That took a lot of patience, adjustments, and fine tuned settings to get the appearance correct on the device. There is also an emulator built-in. Having the Kindle in hand is far better.

Calibre was also a big help. Calibre is more ebook management? It does have some helpful conversion tools for the .epub to .mobi process.

Also during .epub development. I used threepress, http://threepress.org/document/epub-validate, to validate the .epub files as I went.

Focus on the content first not the format was my big lesson. In the end I made the .epub (DRM free) by hand using Notepad++. Just gave me far more control in the look, feel, layout, and fonts. Web developer by trade so it was not difficult to get a handle on.