Capitalize proper nouns, such as the names of people, places, companies, nations, religions, and nationalities. Even common nouns are capped when they are part of a name ("Mississippi River" but "a river"). Brand names are included as well ("Googled" or "Kleenex").
Capitalize derivatives, which are adjectives that depend on a proper noun for meaning. "Rhodesian" is a derivative, so "Rhodesian ridgeback" would be correct, as would "Stilton cheese" and "Christian doctrine." However, if the phrase is now in common usage, then the capital letter isn't needed, as in "venetian blind."
Days of the week and names of months are capitalized. Seasons are not, unless the season is part of a proper name, as in the "Summer Shakespeare Festival."
Names of regions and areas are also capitalized, such as the South (but only when it's referring to a region and not a map direction) and the Main Line. Capitalize the titles of works, such as books, films, short stories, and songs.
Capitalize a job title when it comes immediately before a proper name or in place of a proper name, as in these examples: "Captain Jones is the commanding officer," or "I am reporting for duty, Captain." However, when referring to generic people in that position, use lower case: "We invited captains to the seminar."
When editing, learn the rules of capitalization and be aware they may differ slightly between style guides. When in doubt, look up the word or phrase in a good dictionary. Note any exceptions to the rules on a style sheet.
Now, let's return to "Carnal Lust" and my colleague's latest novel. He knew that "carnal lust" wasn't a proper name or a title but decided to cap it anyway. That's how his protagonist saw the words inside his head, and that's how the writer wanted his readers to see them too.