At the ScrumMaster course I attended we spent some time on how to write user stories and what can go wrong when your are writing them. An important rule is that the user story should always end with a ‘so that…’ . By adding a ‘ so that ‘ you are forced to write a clear story.
However, there is the risk of ending up in a ‘ cause and effect ‘ trap. This article gives some nice pointers on how to avoid that.