A unified approach for analysis of polarization fatigue and size effects on ferroelectric switching in relationship with injection and entrapment of charge at the interfacial layer of the ferroelectric film is developed. The proposed model suggests that an enhancement of the entrapped charge relaxation at the interfaces has a positive impact on both the polarization fatigue and size effects. This concept is implemented experimentally by introducing a thin conductive oxide (RuO2) layer into the top interface of the conventional PZT film capacitors with Pt electrodes. Capacitors prepared in this way show a substantial improvement of fatigue performance and withstand well the thickness downscaling.