If, in addition to being good, you have a story to tell that illuminates the era we live in and won't let the reader sleep, then it'll almost certainly go somewhere. I don't know. Not to the bestseller list, probably. A good agent is, like any good business partner, a blessing. Finally, the large advance.
Large advances are nice.
They're bigger than small advances. Had I been offered a hundred thousand dollars for my first book, I'd have taken it. If offered a hundred thousand now for my fourth, I'd want to see projected figures and promotional plans before deciding. I don't want a business partner who throws money around foolishly and then blames me for the resulting loss, and frankly, I think a hundred thousand would be a foolish expenditure for my fourth book, at this point in my career. If my second and third take off, or if someone comes up with an interesting positioning plan that looks good, then that'll be different.