This question can be anwered quickly: Not must always be capitalized in titles and headings when using title case. While it is a short word, short words are only lowercased if they are either articles (a, an, the), conjunctions (and, or, but, …) or prepositions (in, on, off, by, …). Not falls in neither of these categories—it is an adverb, and so it must be capitalized. Here are a few examples:
Do Not Enter
The Road Not Taken
The World Is Not Enough
If you haven’t done so already, give the Title Case Converter a try. It will turn your input automatically into correctly capitalized titles.
Photo: Steve DiMatteo