Although grace is necessary for man's salvation, we cannot eliminate works entirely from the equation. An acceptance of God into one's life does not mean that we are then free to act as we wish. This is an inherent contradiction. One cannot accept God and simultaneously act against his commandments.
Works are a sign of the acceptance or denial of grace.
When we sin, we deny grace and turn our attention toward evil. In acting correctly, we show God our willingness to accept his saving grace.