You can pay someone from $50 to $150 to format your book for Kindle, or you can do it yourself. All you need is time, patience, and free software. The following is a general outline of the steps to take, and doesn't cover every specific. (The same or similar method may work for the Nook, but I didn't try that yet.)
The tools I used are:
--Microsoft Word (or you can use OpenOffice, free from openoffice.org)
Both Mobipocket applications are free and downloadable from mobipocket.com:
1. Open your document file in Word or OpenOffice.
2. If you haven't finished all of your edits, finish them before proceeding.
3. Save your file in .doc format (not .docx or .rtf) with a new name. Leave your original file unchanged.
4. Make the following changes to your document:
A. Use only a standard font such as Times New Roman.
B. Use tabs for paragraph indents.
C. Use an extra line break between scenes.
D. Put a page break at the end of each chapter.
E. Delete all headers and footers, including page numbers.
F. Replace em dashes with double hyphens, because em dashes don't display properly.
G. Get rid of any unusual formatting.
5. Save your file in filtered html format. (Filtered html allows you to create a web page that is still editable as a Word file, though with limited functionality.)
6. Upload the file to Mobipocket Creator.
7. Upload a cover image to Mobipocket Creator.
8. Click the Build button.
9. View the completed file using Mobipocket Reader.
10. Go to dtp.amazon.com and sign in to your Amazon account, then follow their instructions.