The macroscopic deformation behaviour of amorphous polymers is dominated by localisation phenomena like necking and crazing. Finite element simulations show that the details of the intrinsic post-yield behaviour, strain softening and strain hardening, determine the severity of strain localisations. In order to perform these numerical simulations an accurate constitutive model is required. The compressible Leonov model is, for this purpose, extended to include temperature effects. Experimentally it is demonstrated that by a small increase in strain softening (by annealing of polycarbonate) or substantial decrease (by mechanical rejuvenation of polystyrene), transitions from ductile to brittle and, respectively, brittle to ductile can be realised. An analytical stability analysis is performed that predicts stable or unstable neck growth dependent on the ratio between yield stress and hardening modulus. The extensive simulations and experimental results lead to the conclusion that in order to macroscopically delocalise strain, and thus improve toughness, one has to reduce strain softening or enhance strain hardening, either by improving the intrinsic behaviour of polymers, or by creating an optimised micro-structure.