We describe here theoretically and experimentally a new concept that shows how the geometry of a semiconductor sample can be manipulated to create a large change of the thermoelectric power in a magnetic field. The most effective geometry is one that short-circuits the Nernst-Ettinghausen effect. When the minority carriers have a larger mobility that the majority carriers, this geometrical magnetothermopower (GMT) effect can be designed to freeze out their contribution to the total thermopower. The thermoelectric figure of merit of Bi1-xSbx alloys is greatly enhanced in a magnetic field, due to the intrinsic magnetoresistance of the material. By analogy, the GMT effect described here can be used to design thermoelectric materials with enhanced figure of merit in a magnetic field from other semiconductors. p-type InSb is used here to demonstrate the effect experimentally.