Well, first you have to love writing. A lot of authors love having written. But I enjoy the actual writing. Beside that, I think the main reason I can be so prolific is the huge amount of planning I do before I start to write. I do a very complete, chapter-by-chapter outline of every book I write. When I sit down to write, I already know everything that's going to happen in the book. This means I've done all the important thinking, and I can relax and enjoy the writing. I could never write so many books if I didn't outline them first.