A three-dimensional array of memory elements that reversibly change a level of electrical conductance in response to a voltage difference being applied across them. Memory elements are provided across a plurality of planes positioned different distances above a semiconductor substrate. Bit lines to which the memory elements of all planes are connected are oriented vertically from the substrate and through the plurality of planes. The memory elements can be set to a low resistance state and reset to a high resistance state during standard operation by biasing appropriate voltages on the word lines and bit lines. Prior to standard operation, the memory elements undergo a forming operation, during which current through the bit lines is limited. A forming voltage is applied to the memory elements during forming with a polarity such that the bit line acts as a cathode and the word line acts as an anode, with the cathode having a lower electron injection energy barrier to the switching material than the anode.