Shameless plug: I'm working on a ebook-generating web app that will let you upload a Word/OpenOffice document, have it automagically parsed, then let you tweak it with some useful prettifying algorithms--or by hand--until it looks great. Then you can download it in epub or mobi format.<p>It's a step above Amazon's and Smashword's "Upload and pray" system, and a step below a full-blown ebook editor like Sigil or InDesign. Email is in my profile if you want to get in on an early version.
I've done this myself (hey, look, no self-linking to Amazon!) and the Mac application Scrivener is probably a better starting point. It's free for 30 days (of use, no less), supports much better outline-based editing, and can easily export to amazon's kindlegen.