I first come up with the idea. My main fic, the one that I focus the most on, was just a random idea I had. I had no idea that it would get to be one of the most popular in its fandom on ff.net. When I posted it to be honest I didn't think anyone would read it, and I really had no idea where it was going.
So I talk to my best friend. We sit and talk for hours about fanfiction ideas, what would happen, what would be cool things to happen, etc. We seriously bounce ideas off each other for hours on end. Once we did nothing but that for four hours straight.
One thing I'm known for/big into is character creation. All my characters need to have an intricate backstory, even those that are one time characters. I write a POV switch fic, and if a character has a POV that means that I probably spent time discussing it over with my friend.
When I need help, or lose my spot, I consult him, or remember the last time we talked. I help him with his Naruto fic too. With his help I was able to bounce off ideas, figure out what worked and what didn't and was able to get the main points set in stone.
This guy's been my best friend for ten years now, he's like a brother to me, and he's my beta. :P