A lot of folks put pressure on the foot that's on the up side of the pedal stroke, in addition to the foot on the down side. Try unweighting the foot that's going up while pushing with the foot that's going down.
Once you get used to riding like this, the unweighting will take the pressure off that foot just long enough to keep the numb away. In addition, you'll find yourself riding faster with less effort. It's amazing how much pushing that other leg up will impede forward progress.