I pretty much agree with GregS, but I can see some instances of the verb being followed by a noun or adjective+noun where you have to use "ensure" and not "make sure". Painting road stripes white ensures visibility. - Correct Painting road stripes white "makes sure" visibility. - Incorrect Getting a good night sleep before an exam ensures better grades. - Correct Getting a good night sleep before an exam "makes sure" better grades. - Incorrect

Yes, I cannot think of anyplace one must be used over the other. 'to make sure' sound more active than 'to ensure'. So how I view the two: "Keep your passport handy to ensure going through security goes quickly." "I want to make sure you kids do your homework before playing video games tonight." (Very active involvement)