How To Capitalize a Headline
This is the best approach to writing headlines.
Capitalize the first word of every letter except articles, coordinating conjunctions, and prepositions of three letters or fewer. There’s one exception: Any word that is the first word in the headline or the last word should be capitalized, regardless of its part of speech. So that last headline, in AP style, would leave one “to” lowercase and capitalize the other:
Knowing Which Loved One to Make Your Will Out To
The biggest problem writers have with this simple system is remember that is and it, unlike in, are not prepositions. Is is a verb and it is a pronoun. So they’re always uppercased in AP style headlines. -- Grammar Underground
I literally have this printed on a piece of paper and pinned to a wall. It's from Grammar Underground. It's perfect.