Have you ever noticed that the best writers leave a signature in their words? You can pick up a torn scrap from their book where the nouns have all been blacked out and still know who the author is because of those tiny clues they leave behind.
My roommate in college had a never-fail system for guessing which quotes belonged to Shakespeare. "If the sentence is backwards, and makes no sense, and you don't know who wrote it, that's Shakespeare."
I can tell Terry Pratchett by his word choice*. I can guess my favorite sci-fi authors by the way each character reacts. Sensible and strict is Ky Vatta by Elizabeth Moon, alternating between cautious and insane is John Geary by Jack Campbell, slitting throats and blowing up citadels is Beka Roslin-Metadi by Doyle and MacDonald.
Poetic adventure is the province of Tolkein and Scott Card.
If chalk and skull are used several times per chapter, it's a good bet you're reading the Dresden files.
During NaNo I see this emerging with my friends. LG, notably, is addicted to symbolism. Inky seems to have something shiny and glass in every book. Merc is big on pain and suffering. And I expect Rie's characters will all live Happily Ever After.
For me, it's names. I love to have names that mean something if you know the joke. In Skippy...
Scipio/Skippy - Scipio is the name of a noted Roman general who defeated Hannibal and who was the grandfather to the Gracchi.
Haus Mus- Goes by Mouse... and Mus is Latin for mouse. And Haus sounds like house.
Ali Cattarma- Goes by Alley Cat
Nanoc- The civilized goblin? Read backward reads Conan, as in the barbarian.
I tend to do this in all my writing. In NNN every teacher has a name that ties in with ravens. Bran Corvus (both Latin terns for raven). Munin and Hugin, the ravens of Odin. Morrigan who turned into a raven. And a teacher named Raven.
Maggie Finn in DoJ? Maggie means pearl, but it's also a derivative of Meg and Meg is the short term for Carcharodon megalodon, the extinct shark of titan proportions that was an apex predator in the Pleistocene epoch.
That story also has a character named Portia Will.... Portia from the play Merchant of Venice and Will after the author of the same play.
So, what about you? Do you have some inside joke you like to include in every book? Do you manage to sneak in a reference to someone or something you love? Will some future reader laugh out loud when they find the signature you've included?
*And his footnotes.