At least according to an old writing professor of mine, that's the secret. He had a way of taking your doubts and pulverizing them beyond recognition. His complete equation for finding success as a writer, which he imparted to us during class once, went like this: just keep writing and don't die.
I like that. What he's talking about is persistance and never wavering from the straight path and minimizing the distractions in your life. All of that is in there, but reduced to digestible proportions. And, in a weird way, he's right. And also, the alternative is death and really, once you die, you won't have to worry about writing anyway. Because you'll be dead. Everyone wins!
Of course, this is the same man who wrote some of the most depressing stories I've ever read (chock full of alcoholics and collapsed marriages) and proceeded to cheat on his wife of several decades, with whom he had lots of children, and run away with his mistress. So take it with a grain of salt.
I think that completing stories is also important. Don't just start one story/poem/screenplay/whatever after another without completing anything. I'm guilty of this, but I've decided to stop. Once I get going on a project, I've resolved to see it through to the end, even if it's not going well. Finishing is a huge component of success, in my estimation, so that's important too. Now go forth and conquer!