Use ensure to mean guarantee: Steps were taken to ensure accuracy.Use insure for references to insurance: The policy insures his life.
But this Web site points out that it is not so clear. This is also backed up here (look at bottom of entry).
Sometimes the right word is difficult to find, and there are not always clear answers. But one that showed up recently in a headline at the newspaper at which I work (though not on my watch) was the use of diffuse instead of defuse.
Sometimes a few letters off can land you in a place that you can defend (as in ensure, insure). Other times it cannot.