Write what you know. While I was going school and learning how to write, I stumbled across that thought quite a bit.
Well that's all fine and dandy if you're a fisherman writing about fish, but it's rather limiting when you want to write about magic, dragons, or space ships and computers gone mad. There isn't a person on earth who could say they know what it means to battle a demon while riding a dragon into battle.
However, I do realize that it's an important thought. The problem is, there is a risk of taking it too literally, and thus requiring you to be intimately aware of all the minutiae in the things they're trying to write. Such as learning all the ins and outs of how to care for a horse if you have a horse in the story. It's my new opinion that that is missing the major thing of importance, unless of course the story in question is about raising a horse.
The thing of most importance, and the thing to focus most on when you try to write what you know, is living life. Every story good story is, when you boil right down to it, a human story. All the larger than life settings and awe inspiring settings only serve as containers for the stories of humanity, and of what it means to live.
A story about a fight for survival against all odds against a powerful magical foe is just a story about human tenacity. A story about crossing space and time in search of a lost lover, is just about the driving force of our relationships. The more real the emotions evoked, the stronger the story.
So now when I think of writing what I know, I think of writing what I know of emotion. I think of the need to tell a real story of humanity, of a story that moves us on a level that is understandable on a emotional level, even if we might not know what it's like to ride a dragon.
I think, if given the chance to tell younger me a nice word of advice, it would be to focus more on the heart of the story, and less on the fiddly bits. I am of course still learning what it means to be a story teller, but, God willing, I hope I will be able to keep working at it for years to come.