Computer simulations of lattice systems have not been performed under constant pressure conditions since a volume change move involving the addition or removal of a whole lattice layer has a low acceptance probability. We propose a new method to improve the efficiency of the volume change move and thus allow for the simulation of lattice systems in the isothermal-isobaric ensemble. The method is shown to predict properties for lattice polymer systems in two and three dimensions in excellent agreement with thermodynamic integration results.