Josh Clark is a designer, developer, and author specializing in iPhone user experience. Josh’s outfit, Global Moxie, helps creative companies build tapworthy iPhone apps and effective websites. Follow Josh on Twitter at @globalmoxie. He’s author of two books, including the following tips from his newest, Tapworthy: Designing Great iPhone Apps.

An app’s shelf life lasts exactly as long as it can hold users’ attention. iPhone owners chew through apps, gulping down their content, then tossing them out and moving on. Studies show that the average user never launches an app more than 20 times before abandoning it. Less than 15% of downloaded apps get so much as a glance over the course of a week, and two months after purchase, only one-third of downloaded apps ever get used again.

This may not matter to you if your goal is to build one-off novelty apps; in that case, you might even expect people to launch your app only a few times. Laugh delivered, mission accomplished. If you’re trying to grow a following for your app, however, this is uncomfortable news. According to one survey, nearly half of all apps are downloaded based on a friend’s recommendation. Loyal users spread the word, but few apps ever manage to create a huge fan base. If you’re trying to create a long-term relationship with your audience, your app has to keep giving.

Tools vs. Content

Certain kinds of apps have a built-in heartbeat thanks to the fundamental nature of their key features. Tools that organize personal info (to-do lists, calendars, expense trackers, contacts) keep people coming back. Likewise, utilities that perform a common task (Skype calls, instant messaging, barcode scanning, weather forecasts, notebooks) continue to be useful as long as the underlying task is in demand.

For most of us, tools and utilities still account for the majority of our desktop software. Most folks use computers to work. That doesn’t hold so true for iPhone apps. The majority of apps in the App Store are content apps — games, entertainment, books, references, novelty apps. For these apps, it takes more work to keep the heartbeat thumping. Sometimes that simply means fresh content. News apps, of course, have a bottomless and constantly refreshing reservoir of content, drawing users back regularly for more about the latest political brouhaha or Brangelina update. Non-news content apps have to be a bit more creative, but the challenge is the same.

Unlocking Content Trophies

Games

Games often offer additional levels through in-app purchases, which also has the happy side effect of steering additional cash into the company checking account. But new game levels are about more than just an opportunity to keep playing: They’re markers of achievement — content trophies. That approach might seem unique to games, but other types of apps can offer a similar sense of expansion as achievement, letting you unlock new content as you master various chapters, challenges, or lessons.

In-app purchases and add-ons let users keep the good times rolling . . . literally, in the case of the Skee-Ball-themed game Ramp Champ, which lets you download new ramp themes for free or fee. Skies of Glory, an aerial combat game, lets players upgrade the game’s gear for a fee. Here, a Zero plane will run you two bucks.

Travel

Travel apps are especially road-ready examples that benefit from content that expands and adapts to mirror users’ activities. Lonely Planet Travel Guide, for example, comes with a single city guide to San Francisco, but offers scores of other travel guides and phrase books for purchase inside the app — buy a new guide when you’re headed to a new city. Similarly, OffMaps lets you download maps and guides for offline use anywhere in the world — handy for dodging international roaming fees that otherwise make it ruinously expensive to use the built-in Maps app abroad.

Fitness

This steady unlocking of content is also a staple of fitness apps. CrunchFu, for example, is an app for acolytes of the six-pack ab, a training program to help transform bellies from wishy-washy to washboard. The app gives you a daily training program to do a recommended number of crunches. As you complete each day, you unlock the next day’s program. Each one gets gradually more difficult until you finally gut through 200 crunches at a single go. These steady, game-like accomplishments keep users coming back while also encouraging a safe fitness ramp-up — an effective way to give the app a healthy heartbeat.

Even so, CrunchFu’s content is limited by the fact that its training program has a finish line. After you hit 200 crunches, you’re done. Normally, when users finish an app’s content, they’d amble off into the sunset, never to look at the app again. Even when the training program peters out, however, CrunchFu provides a game to keep its users. The app offers “battles,” head-to-head crunch competitions with other CrunchFu users. No matter where you are in the training program — beginning, finished, or somewhere in between — the app lets you find someone else at your fitness level and challenge them to see who can do the most crunches, earning points as you go.

People Love People

Even as we tap away at our iPhones, off to ourselves and oblivious to the world, we are still irresistibly social creatures. Like a physical place, apps that bustle with the activities of other people feel alive. Community features give an app life beyond its fixed set of content. For some apps, sharing content with friends and seeing what others are saying lets your users provide an ever-replenishing supply of fresh content. Among its many other features, for example, the Movies app by Flixster pools reviews by moviegoers. You can follow the advice of the grand mass of public reviews, or just listen in on what your friends have to say.

Yelp adds similar value by encouraging customer reviews for local businesses, and Amazon, of course, does the same with reviews of its products. In all of these cases, community-driven reviews give extended life to the basic content provided by the app developers.

You don’t have to build this stuff yourself; you don’t even need your own community. Just plug your app into the established social networks, where millions of people are already talking. Facebook and Twitter both provide easy platforms for sharing content from iPhone apps. For games, social networks like OpenFeint or Apple’s Game Center let developers plug high-score leader boards and head-to-head challenges directly into their apps.

Bolt on Features

The apps described so far enhance and extend their value by replenishing their primary content. Another effective, if less elegant, approach is simply to bolt on complementary tools. The idea is to wrap secondary features around your app’s main content to enhance and extend its value. The Typography Manual, for example, is an app for budding graphic designers. It’s essentially an e-book spelling out the history of typography, tracing its letterforms and laying out its technique. [This app is not available in the U.S. iTunes store at time of writing. - Ed.]

While the app does a fine job of dotting the i’s and crossing the t’s of its subject, it remains like any book — a fixed set of words and images. But the app also throws in a handful of tools that are useful to folks who sling type for a living. There’s a font ruler, some font-size calculators, and a comprehensive reference for easy-to-forget HTML codes for special characters. Even after users finish reading the book, these add-on utilities keep them coming back by continuing to provide value.

Think hard about the features that can give your app extended longevity, but don’t just pad the app with features willy-nilly. It’s often useful to design your app for a steady, long-lasting heartbeat. But as you do this, it’s even more important to keep the app simple and focused.

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