elementrolls asked you: I'm one of those people who tends to have a lot of ideas from the get go, but I have a hard time deciding what's just a fun idea (that I might explore in a sequel or another book) and what really needs to go in the story. Any tips on focusing?
One of the pseudo-problems I run into when brainstorming happens when I get a bunch of ideas and suddenly I think I have to either pick this idea or that idea, when most of the time, I can use both ideas. So if you have a lot of ideas, try to see if you can use most of them.
With that said, there is a point where you have to stop yourself or your story will get longer than Lord of the Rings, which isn't a bad thing, but most writers aren't looking to write stories that long.
There are also times when you don't want to use all of your ideas; for example, if you are writing a medieval fantasy, you might not want to throw in time-traveling aliens after you already have a quest story for the main plot. Time-traveling aliens just doesn't fit.
For sequels, a good technique is to look at how you can broaden or deepen your storyline so that the conflict goes farther or deeper than the book before.