The post-yield behaviour of glassy polymers is governed by intrinsic strain softening followed by strain hardening. Intrinsic softening is the dominant factor in the initiation of plastic localisation phenomena like necking, shear band formation or crazing. Removal, or a significant reduction, of intrinsic softening can be achieved by mechanical or thermal pre-conditioning, and is known to suppress necking in tough amorphous polymers like polycarbonate and polyvinylchloride. Here, the effect of mechanical pre-conditioning on the macroscopic deformation of a brittle polymer, notably polystyrene, is studied. As a result of mechanical pre-conditioning, a 30% thickness reduction by rolling, the yield stress is decreased and the intrinsic softening drastically reduced, resulting in a more stable deformation behaviour yielding an increase in the macroscopic strain to break to approx. 20% as compared to 2% in the untreated samples. The effect observed is of a temporary nature, as, due to progressive ageing, the yield stress increases and intrinsic softening is restored on a time-scale of minutes. This indicates that the toughening is indeed caused by the removal of intrinsic softening, and not due to enhanced strain hardening related to molecular orientation induced by the rolling treatment.