Titles bug me in the worst possible way. It annoys me so much not to have one that I can't start writing without one, and then if I can't come up with one right away then I have to come up with a working title, and then spend days agonising over the finished story, trying to think of a better title.
Thankfully, I haven't usually had this problem with fanfiction: the titles have all presented themselves to me almost immediately.
The Thinking Man's Guide to Destroying the World I made up in about three seconds without giving much thought to the story itself - before, in fact, I'd even decided that the world would be destroyed in it. In fact, the story almost came about as a reaction to the title.
A Study in Saffron (posted here as The Rocket Case, a far inferior title) is a pastiche of A Study in Scarlet, at least so far as the title goes. The two stories bear no resemblance to each other except that they feature private investigators.
My Trip to the End of Time, by Pearl Gideon refers less now to the story than when I started; only after finishing did I realise that Pearl Gideon never actually got to the end of time. I'm not sure where it came from.
Coriolanus Rowland's Guide to Pokémon Husbandry is, er, fairly obvious, I think, as are Snow and The Beastman: their subject is outlined concisely in the title.
Crack'd, or How the Love of Seafood Saved Unova is derived grom Tennyson's 'The Lady of Shalott' ('Out flew the web and floated wide:-/The mirror crack'd from side to side'), which I thought fitted a world that resembled ours but was cracked in half down the centre of reality.
A Smell of Petroleum Pervades Throughout was a mistake. It derives from 'a smell of petroleum prevails throughout', the so-called secret of the universe, but when I typed it into the subject bar of my new thread, I found out I'd misquoted it. That meant that I had to work out a way of incorporating the misquote into the story, which in the end resulted in it being a clue towards the importance of letters (i.e. Unown) in the story.
So yeah. My titles are a mixture of haphazard quotations, attempts to catch a theme in a couple of words and 'things what sounded good'.
F.A.B.