The informal rule is a stylistic one. Keep the complement as close as possible.
That really helps me out.
Clearly this is not a lot of separation, and to phrase it "helps out me" would sound awkward and awful.
That really helps out the children who are starving every day in Africa.
To put "out" at the end would simply require the reader or listener to wait too long to parse your verb as a phrasal verb.
To sum it all up: it's a judgment call.
To sum up everything I have stated in this response: it's still a judgment call.
There are no special rules for rarely used prepositions. According to the Chicago Manual of Style, all prepositions should be lowercased (except when they are stressed or occur as the first or last word). But I hardly ever see this style used. The AP style is much more common. According to the AP stylebook all prepostions containing three letters or fewer should be lowercased, while the Wikipedia Manual of Style says all prepositions containing four letters or fewer should be lowercased.
However, the Wikipedia rules are not enforced, and as a result the capitalization on Wikipedia is a mess, and often in violation of the style manual (as you had observed). I think the reason for this is that the AP rules are dominant in real life, so this is what people are used to (i.e., what “looks right”). This explains many of the cases that you listed: down, over, unto, upon etc. are all capitalized according to AP style.
So regarding your examples, it depends on the style guide you choose whether the four-letter prepositions should be lowercased. Three- or two-letter prepositions like on, off, out or up should always be lowercased (if they are really used as prepositions and do not occur as first or last word). Yet should also be lowercased, if used as a conjunction.
There is no difference between book titles and song titles. The AP style for instance says that its composition title guidelines apply to book titles, movie titles, opera titles, album and song titles, radio and television program titles, and several more.
Best Answer
The relevant rule in Chicago is rule 3 in section 8.167 of Chicago Manual of Style, Fifteenth Edition (2003):
Chicago acknowledges that this and its other five rules for handling title-case headlines are arbitrary. It calls its rules "pragmatic rather than logically rigorous but generally accepted"—presumably meaning generally accepted by publishers who pick and choose among Chicago's style recommendations in assembling their own house style.
In any event, the out in the headline "This Is How He Finally Got the Hell Out of Mexico!" is functioning in the same way as the out in Chicago's example, "Taking Down Names, Spelling Them Out, and Typing Them Up": It forms a phrasal verb (in one case with "Get"; in the other with "Spell") with a verb that is separated from it by one or more intervening words (in one case "the Hell"; in the other "Them").
That the final phrase "of Mexico" is irrelevant to the status of out as part of "Get Out" is clear if you remove "the Hell" from the equation—which is a legitimate shortening of the title because the person in in the title isn't removing the Hell from Mexico, but rather his own person. In that simplified case, Chicago clearly prescribes this punctuation:
The only situation where Chicago would endorse lowercasing the o in out is if the word were functioning strictly as a preposition, as (arguably) here:
I am confident that Chicago would recommend this capitalization of the OP's title: