Now we come to what I think is the most important rule of all for a writer: Don't flinch.
Everything you see and experience can be used in your writing, but you must have your eyes open to see it. Be aware of your surroundings. Listen to what others say, and how they say it. Don't talk much yourself, but draw others out on what they think and believe. Be open to new experiences, different ways of doing things. Travel. Become friends with different types of people. Even if it's hard, even if you're shy, do it for your writing. Don't flinch.
Sometimes you'll see something happening that's wrong. If you can alter it, by all means intervene. But maybe you can't really do anything about it, or your interference would only make things worse. If you're a writer, your job isn't over in that case. Keep looking. You can use it later. When others learn of it, they may have the means to act. Whatever you do, don't flinch.
When you start writing, and you're putting your thoughts and ideas on paper, you may come to a part that's emotionally difficult. Maybe your characters will say ugly things, or uncomfortable events may transpire. Let them. This is the part that others need to read. They need to know others have thought those thoughts, or felt those feelings, or had those things happen to them. The ugliness and discomfort need to be out in the open. If it's ugly and needs to be killed, how can you do that if you can't even see it? But sometimes, something you thought was ugly turns out to be beautiful once you really look at it. You have to see it to know. Don't flinch.
Perhaps you're writing escapist fiction. Shouldn't you leave the ugly and uncomfortable out? After all, people sometimes just want to read something for fun without all that real world stuff in there. You'll have to use your judgment, but I would point out that some of the world's great escapist literature had a lot of uncomfortable truth. Think of Huckleberry Finn, on one level a boys' adventure story, on another a penetrating look at attitutes towards race. And even in escapist fiction, characters still have to follow their own nature. Plots still have to unwind plausibly. Sometimes that means they don't quite go where you want them to. Don't flinch.
Maybe you're writing a book for children. Of course there is material that's inappropriate for kids. That's why fairy tales disguise uncomfortable truths in magic. Once you break it down, is there any story, anywhere, harder and more clear-eyed than Hansel and Gretel? Perhaps Lolita, but not much else. And it's a fairy tale! Even when writing for children, don't flinch.
Once kids are older, they can handle a lot, probably more than you think. I well remember the smart kids in the seventh grade passing around Flowers in the Attic. That book was truly lurid, with themes of bondage and incest, but we ate it up. Probably not the healthiest thing for us to read, but we weren't corrupted. If anything, it provided us a few pieces in putting together the puzzle that was sex. It doesn't matter that it was trashy, we knew that, and knew it wasn't something emulate. We were wide-eyed, and willing to consume anything that might help us understand. Better were the YA novels of, say, Judy Blume. Heavy, sexual subject matter, but treated with sensitivity. That's how you should do it. But whatever you do, don't leave it out. Don't flinch.
In the post on writing breakout novels, I think I mentioned how that book describes debut novels, and novels by authors who haven't broken out, as feeling small. I believe one key to overcoming the smallness is being willing to turn the light of fiction on those dark corners that many are afraid to peer into. Even if monsters lurk there, even if ghosts pop out, whatever you do, don't flinch.