Smartphone owners often hand over their device to another person for temporary sharing, such as for showing pictures to a friend or entertaining a child with games.  This device sharing can result in privacy concerns since the owner's personal data may become vulnerable to unauthorized access.  Existing solutions have usability problems and neglect human factors of sharing practices.  For example, since device sharing implies trust between people, explicitly hiding data may signal mistrust.  Besides, an owner may fail to enable a sharing-protection mechanism due to forgetfulness or lack of risk perception.  Therefore, we propose device sharing awareness (DSA), a new sharing-protection approach for temporarily shared devices, which detects a sharing event proactively and enables sharing protection subtly.  DSA exploits natural handover gestures and behavioral biometrics for proactive sharing detection to transparently enable and disable a device's sharing mode without requiring explicit input.  It also supports various access control strategies to fulfill sharing requirements imposed by an app.  Our user study evaluates handover detection over 3,700 data clips (n=18) and comprehensive device sharing processing over 50 sessions (n=10).  The evaluation results show that DSA can accurately detect handover gestures and automatically process sharing events to provide a secure sharing environment.