Resistive memory materials and devices (often called memristors) are an area of intense research, with metal/metal oxide/metal resistive elements a prominent example of such devices. Electroforming (the formation of a conductive filament in the metal oxide layer) represents one of the often necessary steps of resistive memory device fabrication that results in large and poorly controlled variability in device performance. In this contribution we present a numerical investigation of the electroforming process. In our model, drift and Ficks and Soret diffusion processes are responsible for movement of vacancies in the oxide material. Simulations predict filament formation and qualitatively agreed with a reduction of the forming voltage in structures with a top electrode. The forming and switching results of the study are compared with numerical simulations and show a possible pathway toward more repeatable and controllable resistive memory devices.