The viral game Angry Birds has had over 500 million downloads and earns over $1 million a month just from in-game advertising. Their 2011 revenue was around $100 million total. While this is an extreme example, it does illustrate just how powerful a viral iPhone and/or Android app can be.

There are hundreds of millions of smartphone devices in circulation today. Yet few businesses make the crossover between their primary business and the world of smartphones.

By creating a viral smartphone app for your audience, you can leverage your existing brand and increase both its usefulness and its virality.

People who look through a friend’s iPhone or Android phone will see your app there. People tell friends about useful apps. You can also put “Share” buttons within your apps and have people post your app to Facebook, all from their phone.

So how do you create a viral app?

==> Create a Unique User Experience

The experience people have needs to be unique. It should either be entertaining or it should solve a problem.

Angry birds, for example, doesn’t solve a problem. However, it’s highly entertaining. There’s no other user experience that’s quite like it.

By contrast, there are many photo editing apps that also do really well. These aren’t entertaining, but they’re very useful. Many of them solve problems (like removing blemishes) that no other apps solve.

==> The Experience Must Be Repeatable

The app you create needs to be an app that’s accessed on a regular basis. You can’t expect a one-time use app to go viral.

This is one of the reasons why apps like FourSquare went viral so quickly. Every time someone visited a physical business location, they checked into that location. They were using the app constantly, which kept it in their minds. Also, friends see them using the app and ask about what it is.

==> Sharing Must Be Built Into the App

Finally, sharing has to be built into the app’s functionality. An app that’s useful but isn’t easy to share is much less likely to go viral than one that promotes sharing.

One of the most common ways to promote sharing is through competition. In FourSquare for example, people compete for badges and mayorships. This competition causes people to check into locations more often and share more often.

==> The “Shots At Bat” Mentality

Think of building an app as stepping up to the plate. You might hit a home run, but you might strike out. The best way to hit a home run is, of course, to hit the ball as many times as possible.

Don’t be discouraged if your first app doesn’t go viral. Sometimes it takes a few tries. Look over what you did right and what you did wrong and take another shot. Eventually, you’ll hit a home run.