A method is provided for retaining a stack of sheets in a post-processing unit which improves retaining capabilities of a retaining element while retaining a stack of sheets. This method comprises the steps of forming a stack of sheets, providing a wire from a wire source, the wire being plastically-elastically deformable, selecting a retaining element type from one of a clip element type and a stapling element type, determining a wire deforming program for forming a retaining element of the selected retaining element type, forming the retaining element according to the wire deforming program by bending the wire at a number of positions along the wire and cutting the wire, and applying the formed retaining element to the stack of sheets, thereby retaining the stack.