Find seriously skilled people to build your app
To create something that looks as slick and functions as smoothly as the other big hitters in the app store, you’ll need to hire top design and developer talent, and convince them to stick around after launch to help work out any kinks or glitches -- which will most definitely crop up. Often times that means ponying up a fat check up front, offering equity, or both.
Beta test the crap out of it
Once you have an operational version, you’re going to want to run it through every potential user scenario. Push it to the limit trying the things you expect people doing with it, and things you'd never imagine they would. Get as many friends and acquaintances as possible to download and test it out, too. There’s nothing more embarrassing than pushing some half-baked version of an app that crashes every five seconds.