NaNoWriMo Tip
Here’s a tip for everyone doing NaNoWriMo.
Say while you’re working on your book you get an idea for a decent 2-4K short story, but are uncertain whether you should take a break long enough to write it.
Now this isn’t an instance of “I think this’ll make a good story”. No, this is the story has more or less bubbled forth fully formed out of the froth of your NaNoWriMo-fevered brain, and since you’ve been putting in long hours pounding the keys you KNOW you can write this story, polish it, and get it out the door in 24 to 48 hours.
Well, if that’s the case, and you’re at all interested in actually completing some decent fiction this month, then take the break and write the story.
Your novel draft can wait. Inspiration won’t.
Yesterweird 
This is excellent advice for the fully formed story that is pushing its way out voice and language and all. For the other variety, the “I think this will make a good story” premise or plot line, I recommend not even taking notes. Completely ignore the idea. If it’s truly compelling as a concept, it’ll return to you after finishing the novel. This will have the added benefit of separating the wheat from the chaff. Ideas always popping up, time remains a writer’s chief antagonist.