Depending on your financial resources and the type of app you're building, a big launch may be the way to go. The idea of a big launch is to make a big splash and drive as many downloads in the first week as you can in an attempt to get onto the App Store leaderboards. Once on the leaderboard, if people like your app, then you'll may be able to maintain your position.
Getting onto an App Store leaderboard can easily mean thousands (or tens of thousands in games and social categories) of downloads per day. That is incredibly valuable free marketing - which is why people spend so money to get there!
The best way to do the big launch is to get Apple to feature your app. This sounds difficult, but I know multiple people and companies that have had apps featured at launch - even one built by a sole developer.
Here are some tips for attracting Apple's attention:
Know someone at Apple. Seriously, the App Store team is pretty big, and working personal connections is absolutely the best way to get featured. This is obviously easier for publishers located in Silicon Valley.
Show off a new way or unique way to use the device. Apple makes the vast majority of its money selling iPhones and iPads, so they LOVE apps that show off the power of the hardware (as long as those apps don't run on Android). Notable apps using this strategy included Flipboard (iPad as a reading device), Bump, and Frontback.
Do something different in a less popular category. The App Store is full of incredible quality games. But get out to the smaller categories like Business or Productivity and there's lots of dreck to wade through. Nonetheless those categories get their own featured lists, so if you build a good app, with a good twist, in a less popular category you've got a good chance to get noticed.
Quality, quality, quality. Apple rewards people that put as much effort and attention to detail into their apps as Apple thinks they should! High quality is essentially a requirement to getting featured. Note that it also makes app approval a lot easier.
Beyond getting featured by Apple, you need to get "big press" if you want a big launch. Give Techcrunch an exclusive on launch day. Then approach 20 bloggers and ask them to review your app. Contact Robert Scoble and offer him a "sneak peak".
Finally, you can buy downloads and reviews, from 3rd party services. This is great if you have the money, and believe me, many of the top app publishers do this regularly. For the little guy, you need to think a little more guerilla.