Here's our rule of thumb: if you're reading a sentence by Douglas Adams and it seems pretty simple, then watch out, because he's probably going to twist it in some way. The classic example of this is Adams's description of the Vogon ships: "The ships hung in the sky in much the same way that bricks don't" (3.65). We love that sentence because (a) it calls up an image of bricks floating in the sky, which helps us picture the Vogon ships even without describing them; and (b) the sentence describing the Vogon ships ends in a way we wouldn't expect. We might expect there to be some majestic description of flying space ships. But instead, Adams compares them to something that's really just one level above dirt in terms of how interesting it is.
Adams loves these little twists, especially if they are anti-climaxes. He builds us up so we might expect something great, and then he lets us down—and he lets us down hard. For instance, when Arthur meets Zaphod, he declares that he already knows him and even names him: "We've met, haven't we Zaphod Beeblebrox—or should I say ... Phil?" (13.43). Now, the name "Phil" is a little bit of a letdown when we're just getting used to Zaphod's crazy name. In addition, we might expect Arthur to know Zaphod from some crazy event, but it was just from some party that they happened to both go to.
This love of anti-climactic twists works both at the sentence level and at the larger plot level for the story. For instance, we might expect the destruction of Earth to be the result of some giant war, when it's really just a bureaucratic construction crew. We might expect the last survivor of Earth to be a hero—but it's just a guy who happened to befriend an alien.
The Reworked Cliché
Another move Adams sometimes makes is to rework a cliché. For instance, when we say "I'd trust him to the end of the Earth," we usually mean that we trust this person completely. But when Ford uses this phrase to describe Prosser in Chapter 1, he means something else: he would trust Prosser for "About twelve minutes" (1.161), since Ford knows that's about when Earth will be destroyed.
For another example, when Arthur saves the Heart of Gold from nuclear missiles, Zaphod tells him how wonderful he was to get the Heart of Gold out of danger, and Arthur responds, "well, it was nothing really…" (18.15). "It was nothing" is usually just a polite thing people say so to avoid bragging. But Zaphod doesn't recognize it as a form of politeness—he takes it seriously and tells Arthur to forget it if it was just nothing.
Now, those sort of reworked clichés are funny to us. It's especially fun when a character in the book takes a cliché seriously; it brings attention to the fact that a lot of what we say is actually kind of funny. It's also kind of hilarious when the reworked cliché, taken seriously, actually tells the truth about a situation—like when Ford correctly states that he'll trust Prosser for the twelve minutes left until the end of world.