Did you create the post here http://wordpress.com/#!/post/ where the auto-save does not work?
WordPress.com has an autosave feature and you may be able to recover an earlier post revision. Go to Posts > All Posts, hover over the title of the post, click Edit and check the Post Revisions module for previous saved versions of the post. http://en.support.wordpress.com/posts/post-revisions/
If you're not seeing the Revisions module while on the editor screen, click Screen Options (top right corner of your Admin page) http://en.support.wordpress.com/screen-options/ to activate it. Then select the post and page modules to include them. http://en.support.wordpress.com/modules/#list-of-modules Then you ought to be able to go here > Dashboard > All Posts > Posts and locate the post and recover it from post revisions.
If you cannot locate the post for locating and restoring a post revision, and it's not in your Trash folder http://en.support.wordpress.com/trash/ then you will not be able to recover it. http://en.support.wordpress.com/trash/#restoring-from-trash
Create your posts on the dashboard of your own blog where the auto-save works
Dashboard > Posts > All Posts > Add New
Or use an offline blog editor so you always have backups on your own computer http://en.support.wordpress.com/xml-rpc/