This is the general rule for backing up data -
For more information about the 3-2-1 rule check out this post from Windows IT Pro.
Personal Archiving from the Library of Congress - http://www.digitalpreservation.gov/personalarchiving/
File Types from the National Archive - http://www.archives.gov/preservation/products/definitions/filetypes.html
Preservation Information from the National Archives - http://www.archives.gov/preservation/
Digital Imaging Tutorial from Cornell - https://www.library.cornell.