If you don't want to litter your app with advertising, then the "freemium" model -- basic features everyone can use, a paid tier for customers who want more advanced options -- is one option. Rather than offering a complex set of features, try and work out if you can charge for one or two premium option.
That's the approach adopted by presentation tool Prezi, which has managed to sign up more than 50 million users. Prezi itself is free to use. Paying $US6 a month gets you just one extra feature -- your presentations are private. If you don't pay that, then any other use can browse and view your creations. That in itself is enough incentive for many business customers. For $US14 a month, you can also access and edit presentations offline on PCs and Macs.
Drew Banks, head of international at Prezi, told Lifehacker during a recent visit to Australia that the approach had two advantages: it's easier for customers to understand the pricing model, and it's much simpler for developers since the core feature set remains common across all users -- there's no need for separate free and premium code forks.
While the feature that you make people pay for will vary depending on the app, it's a model worth considering. Whatever features you charge for, updates are still something you need to make regularly -- you'll increase your profits that way.