  Extra neutral gauge bosons suggested by models beyond the standard model can
indirectly show up in $e^{+} e^{-}$ collisions through off-resonance deviations
of various physical observables from the corresponding standard model values.
We considered leptonic observables and studied the dependence of the deviations
on the polarizations of the positron and electron beams. We showed that, for a
given model, the magnitude of the deviation of a given observable can attain
its maximum value if the polarizations of the positron and electron beams are
properly chosen. We determined, for a given model, a single set of beam
polarization so that if this set is employed in measuring all considered
observables, it produces the highest extra gauge boson discovery limits.
