When to Capitalize the Words in a Headline

Thank you, ProofreadNOW.com for this. I was just wrestling with this for a client who is always having me post content from many sources. Many of the sources don't have a clue about punctuation or caplitalization.  You came to the rescue. Read on. Perhaps this will help you with your headline capitalization questions.

Consistency is the key in the end. 

If you're in the newspaper business, public relations or advertising, you know how to properly capitalize headlines. But people writing white papers, press releases, brochures, and even résumés need to know what's right and what's wrong in order to retain the respect and admiration, to say nothing of the trust, of their readers. 

Most style guides call for lowercasing prepositions, articles, and many conjunctions. But there are lots of extenuating circumstances that call for uppercasing those words sometimes. Read on, but first:
A preposition is a word that could describe your relationship to a cloud: you're in the cloud, under the cloud, above the cloud, around the cloud, by the cloud, before the cloud, after the cloud. These italicized words are prepositions. 
– The articles are the, a, and an — they point out things: the boy, a man. 
Conjunctions join things: and, or, nor, while, etc. 
The Chicago Manual of Style says to always capitalize the first and last words of a headline, no matter what.
Lowercase prepositions, regardless of length, except when they are stressed (as through in A River Runs Through It), are used adverbially or adjectivally (as up in Look Up, down in Turn Down, on in The On Button, etc.), are used as conjunctions (such as before in Look Before You Leap), or are part of a Latin expression used adjectivally or adverbially (e.g., De Facto, In Vitro, etc.). CMS specifies lowercasing the conjunctions and, but, for, or, nor. Always lowercase to and as
Mnemonics That Work Are Better Than Rules That Don't 
Singing While You Work 
A Little Knowledge Is a Dangerous Thing (is is a verb, and verbs in headlines are always capped) 
The Water Skier as Bride 
Tired but Happy 
Traveling with Bosco, but A Good Dog to Travel With 
Progress in In Vitro Fertilization 
Voting For and Against the Tennis Court Proposal
The Gregg Reference Manual and the AP style guide say to capitalize all words with four or more letters, including four-letter-plus prepositions. You might find this rule more attractive especially with regard to long prepositions such as through and multisyllabic prepositions such as around and underneath.
Standard newspaper rules call for capitalizing the first word in every line of a column headline that is forced to wrap onto two or more lines. For example, 
Federal Tax 
Dollars Stolen 
In Boston's "Big Dig" 
Always refer to your chosen style guide and be consistent.