I started "An Ounce of Prevention" with a title taken from a common... aphorism? Proverb? Whatever. When I began turning it into a chaptered story, I stuck with that theme, so each chapter title is half of an English saying. And there is, in fact, a rationale to their relationship with the chapters -- I didn't just pick them out of a hat.
1. An Ounce of Prevention (is worth a pound of cure)
-----A little caution at the beginning is better at solving a problem than huge, flashy heroics after everything has gone to hell; i.e., a ninja would deal much better with Tom Riddle than Ginny and Harry did.
2. When in Rome (do as the Romans do)
-----Follow the laws and customs of the land in which you find yourself; it saves trouble. In other words, if you're a ninja, act like a ninja.
3. Imitation Is (the sincerest form of flattery)
-----Tom is fishing for information. Fish back.
4. Birds of a Feather (flock together)
-----There are interesting parallels between Sakura and Anko, as well as between Sakura and Tsunade. (And Anko and Tsunade, but I didn't go there much in this story.)
5. Assumptions Make (an ass out of you and me)
-----I mangled this quote slightly. (It's really more like "Don't assume; it makes an ASS out of U and ME." It's a spelling joke.) But the point stands; Sakura must be cautious with Tom and Anko.
6. Too Many Cooks (spoil the soup)
-----It's hard to work with people jogging your elbow. Also, it's hard to write a Sakura-talks-to-Tom scene with another person in the room, commenting.
7. Time Flies (when you're having fun)
-----Ah, irony.
8. If at First You Don't Succeed (try, try again)
-----Frustration is a lesson.
9. Idle Hands (are the devil's playground)
-----If you aren't doing something useful, chances are you'll get up to mischief; i.e., Sakura takes a day off and ends up less relaxed than before.
10. Can't See the Forest (for the trees)
-----Step back and look at the big picture.
11. Never Put Off for Tomorrow (what you can do today)
-----Procrastination is bad, children. (This, by the way, is a vicious, evil lie. *grin*)
12. Even a Stopped Clock (is right twice a day)
-----Or, the Leaf-nin hit on the right conclusion, even though they don't really understand magic or the nature of a horcrux.
13. Don't Judge a Book (by its cover)
-----This title was almost too apt; it verges on cutesy. Just because the diary is evil doesn't mean that Tom is evil, right? He could be an innocent victim trapped in a soul-sucking prison, right?
14. Put Your Money (where your mouth is)
-----If you're going to protest Tom's innocence, prove it.
15. Curiosity Killed (the cat; satisfaction brought it back)
-----Sakura gets into trouble. (I was initially planning on doing all the in-Tom's-mind scenes as only two chapters, one titled "Curiosity Killed" and the other titled "Satisfaction Brought," but the scenes expanded on me and I only used the first title.)
16. Fool Me Once (shame on you; fool me twice, shame on me)
-----Sakura does not get taken in by Tom.
17. A Miss Is as Good (as a mile)
-----Hello, fight scene! Tom, you are so outclassed.
18. He Who Laughs Last (laughs best)
-----Then again, Sakura, you can't consider yourself safe until you've escaped.
19. Once Burned (twice shy)
-----Sakura is not having a good day.
20. Three Can Keep a Secret (if two of them are dead)
-----You can't keep a soul-sucking diary locked away and not expect someone to find it. Narrative causality practically demands the secret be revealed.
21. If Wishes Were Horses (beggars would ride)
-----It would be nice if we could solve every problem, but life doesn't work like that.
22. What Doesn't Kill You (makes you stronger)
-----Unless it cripples you, of course, but we're speaking metaphorically. And Sakura is strengthened in her determination to help her teammates.
23a. The Road to Hell (is paved with good intentions)
-----Brilliant job, Sakura, telling Lucius Malfoy exactly how the diary works.
23b. All Roads (lead to Rome)
-----Actually, this is another iteration of the previous saying -- only here it's Ginny's good intentions leading to disaster -- but I'd already used that title. So we're left with the idea that CoS is fated to happen. Depressing, eh?