Workbench Moderation - intermittent save / autosave without new revision
You can do it by using these modules :

Save Draft


  Save Draft adds a 'Save as Draft' Button to the node_form for content
  types, allowing the user to click the 'Save as Draft' button to save
  the node as a draft.


Draft


  The Draft module provides the ability too create drafts of existing
  nodes as well as new nodes before you have saved them.
  
  The module utilizes the jquery.fields plugin to serialize all of the
  information and then the information is stored in a database row
  serialized.
  
  The module provides the ability to autosave the content without
  the user having to hit the Save as Draft button on the form.