1) Get it in early. Either on new apis, or new hardware. Apple tends to feature apps that feature their new apis nicely. One of the reasons Instagram was promoted to heavily was that it was released at the perfect time -- the iPhone 4 came out and had a fantastic camera. Instagram drove iPhone sales, so Apple did what they could to promote it even more. I also know someone who made a good, solid iPad app at release and now they make a lot of money off it without needing any real support.
2) Know someone at Apple Developer Relations and help get your app noticed. I've heard stories of seed funds and VCs having connections with the developer relations team at Apple. They can help put it on the front page, which will bring you a lot of downloads. It won't make you a king, but it's a big deal.