Om Malik over at GigaOm has put together a list of 10 commandments for a successful freemium app.

1. Make sure that the usefulness and engagement of the app are in perfect sync.

2. Make your user value proposition a simple one.

3. Focus deeply on one single domain.

4. Clearly define what is free and what is paid.

5. Build a subscription service into your application.

6. Encourage your customers to use your application often, for the more they use the application, more likely they are to establish a relationship with your company and that means you can sell them something new (or an upgrade) in the future.

7. Data is the ultimate lock in. The more data that is stored inside the application, more difficult it is for customer to switch, because of the extra effort involved.

8. Free is free marketing. Instead of advertising, the service should sell itself.

9. Make sure what you started offering for free, remains free.

10. Ensure your app works across all platforms, including devices (such as the iPhone) preferred by your likely customers.

Photo by Twon.