Now, when most of you think of multi-tasking, you probably think of consciously doing two things at the same time. Like talking on the phone and making dinner, or rubbing your tummy and patting your head. All good examples, but not what I'm thinking of.
What I want to talk about is the kind of multi-tasking where you prime your subconscious, and let it do the heavy lifting, while you go on about your life. You already do this all the time without realizing it. It's only a small step to doing it on purpose.
One way to utilize this is to fix your mind on a particular story you might be working on, just before you go to sleep. This works best with stories that you are stuck on, or unsure how to finish. Think about the characters, let your mind roam. Sometimes, you might have to jump out of bed and write the answer to your narrative problem, if it just pops into your head. Other times you might dream the answer, or have it come to you at a seemingly random point in your day.
I call this effect the "Internal Churn". Like the opposite of a woodchipper. You give your subconscious a bunch of fragments, and it will put together a narrative. Sometimes you may get so many ideas that you have to discard some, sift through them for the right one. Your subconscious will bust ass to be creative for you, if you let it.
I try to keep myself thinking about creative things throughout my day. I take a moment here and there to reflect on the stories I am writing, or want to write, and let the thoughts stew, then let go. I nearly always get far better return than if I continued to beat my head against a wall, trying to force creativity.
So - Focus, Fix, and Let Go. Easy peasy.
A few good references - Walter Jon Williams's Aristoi, Richard Bach's Illusions