When you have an idea, a cool snippet of text, and joke, the honey-dewdrops of grace from the muse, the epiphany that will allow you to tie your novel together in such a unique way the Pulitzer Committee will rise up as troop of meerkats scenting something new on the wind, write it the fuck down.
Just stole a joke from one of my co-workshoppers. She said it just before we went to dinner and I though, "oh, no, I don't need to write that down now. I'll remember it when we get back." Yeah, took me five minutes and I'm damn glad I remembered it.
This is a lesson I keep having to learn over. Strike while the iron is hot.
I think I read this first at the end of Zen and the Art of Motorcycle Maintenance that you should pay attention to your ideas. Even if you think, "That's not what I'm looking for" or "That's not what I need," write it down. Sometimes those ideas are just the older sibling of the idea you need, but if you don't clear the way, that other idea won't come out. And if you ignore ideas your brain/muse/universe/whatever will learn that lesson and simply stop giving them to you. So, even if that idea may not be what you need, get it out of the way so the rest can come out.
It's like plumbing. If you stop up the drain, the water stops flowing.
I know this. I've experienced the long deserts of ideas when I haven't obeyed this simple rule. I should know better. I'm sorry, Muse. You keep telling me, I'll try to remember this time.