Participants never act completely naturally in a lab setting, so consider testing your product in the ‘wild’. This can be particularly valuable when real-life disturbances are important for your design, as is the case with mobile apps.
Recruit users and ask them to use your device for a period of time. In a field trial, you cannot easily adapt, so preparation is key. Make sure your prototype is robust enough to be used in the field. Make a detailed list of evaluation questions and think about how and when you can measure them (logs, questionnaires, interviews).
Field trials are particularly suitable for mobile and social solutions that are hard to test in the lab. A particular kind of field trial is a beta-test, which is common practice in the game industry for example.