Quelle: The Chicago Manual of Style
The conventions of headline style are governed mainly by emphasis and grammar. The following rules, though occasionally arbitrary, are intended primarily to facilitate the consistent styling of titles mentioned or cited in text and notes:
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Capitalize the first and last words in titles and subtitles (but see rule 7), and capitalize all other major words (nouns, pronouns, verbs, adjectives, adverbs, and some conjunctions— but see rule 4).
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Lowercase the articles the, a, and an.
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Lowercase prepositions, regardless of length, except when they are used adverbially or adjectivally (up in Look Up, down in Turn Down, on in The On Button, to in Come To, etc.) or when they compose part of a Latin expression used adjectivally or adverbially (De Facto, In Vitro, etc.).
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Lowercase the conjunctions and, but, for, or, and nor.
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Lowercase to not only as a preposition (rule 3) but also as part of an infinitive (to Run, to Hide, etc.), and lowercase as in any grammatical function.
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Lowercase the part of a proper name that would be lowercased in text, such as de or von.
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Lowercase the second part of a species name, such as fulvescens in Acipenser fulvescens, even if it is the last word in a title or subtitle.