Asymmetric switching properties were measured and quantified in sol-gel derived PZT thin films. The hysteresis asymmetry is attributed to the presence of an internal bias field that has been correlated with the self-polarization during fabrication. Annealing the sample under a remanent polarization with the polarity opposite to that of self polarization can yield a partially compensated, completely canceled, or even a reversed internal bias, depending on the annealing temperature. Three dominant processes associated with charge trapping at elevated temperatures are clarified and correlated with the annealing effects. They are aging, de-aging, and the redistribution of trapped charges. A model using simplified domain configurations is proposed to explain the observed annealing effects on hysteresis asymmetry and switchable polarization.