Or, in other words, a crazy little seed.
Now I know that people tire of hearing it--every author says it--but it's true: Ideas come from everywhere. It's not really the best advice for one reason--they give you no tools to access the amazing potential of "everywhere". Some people don't need to be given these tools because, in a combination of luck and practice, they have these tools ingrained. These are those people that give these wild sounding answers about getting ideas for a tribe from this really cool looking beaded necklace.
I don't know if anyone wants them, but I'll lay down a mediocre attempt to give y'all these tools.
First: you have to stop punching your muse in the face. Really, that's what happens. Muses, resilient little things that they are, will constantly yell things at you. Of course you need to filter it, but you may want to try seeing just where that idea tries to go. I once got a complete plot (yet to be written) from an apple. Just an apple. I let my mind run around and *poof* I ended up with a world and a storyline with bunches of subplots all in about half an hour. An apple. Follow the lines--it works.
Second: do not fear encyclopedias. All of these connections that lead into plot and story seeds (or at least a lot of them) come from what you learn through research. I go back to the apple, because I think about apples a lot. When i think apple, the first thing that pops into my mind is Aphrodite. Why? Apples are traditionally held as sacred to Aphrodite. From there, the world opens up to you. Along with cold hard facts, you also need those unique connections, like ravens and foot diseases. I have this in my mind thanks to my grandmother and a series of complex stories. Suffice it to say, without numerous connections like those in your head, pulling story bits out of thin air is going to be considerably more difficult.
That's it. That's how you do it. Have connections and start listening to your muse.