I'm trying to hunt down a story (I think it's very early SF) about a character who tries to convince his friend that he will be reincarnated. He trains himself to trace a symbol--a Greek letter, can't remember but either "phi" or "theta"--so automatically that he does it all the time, unconsciously, telling the friend that this symbol is how he can be identified in his next life. The character dies, and one day the friend is walking down the sidewalk and observes a snail, slowly tracing the symbol with its slime as it crawls. It was a very chilling ending, as I recall. But I can't remember the title or author.
This is "The Sign" by Lord Dunsany. It's described on a message board as follows:
Another of Jorkens' tall tales from the billiard room, this one concerning the transmigration of the soul. Horcher, a walking superiority complex and habitual snail-stomper, promised Jorkens that, when he died, he'd leave a sign to inform him of his elevation to God-like status. In the event, it doesn't quite work out for him.
I read the story myself and remember the details from your query that aren't in this description: Horcher training himself to draw the phi sign automatically, and the snail drawing the sign after Horcher's death. (If I recall correctly, Horcher doesn't literally expect to attain God-like status, but he does expect to be a Very Important Person.)