While in the creation process, if you tap the "x" or ">" in the upper right and left corner of the screen, you'll see a prompt asking if you want to save a draft or discard the post. Tapping "Save" will create a draft of your post which you can resume editing by going to the "My Business" tab in the lower right corner of the home screen and then tapping on "Drafts." 

Please note that selecting "Discard" will delete the post completely!