It’s not a science. It’s an art and a sometimes it’s a craft. The most important thing (and I know I say this a lot but it’s true, or at least it’s true for me) is finishing things, because that’s when you find out if they worked or not. The rest of the time it’s just hoping. And if you stop writing when a promising beginning runs out of steam, maybe you need something more in the planning stages. Or maybe you just need to soldier madly onward and see what Chance and Necessity (the mother, it must be remembered, of invention) provide.