  We discuss analogies between the charge- and spin response functions of
itinerant magnets. We show that the spin-analog of the charge stiffness is not
given by the usual spin stiffness rho_s, but by the dynamic spin stiffness D_s,
which is obtained from the dynamic spin conductance G_s (omega) in the limit of
vanishing frequency omega. The low-frequency behavior of G_s (omega) is used to
define ideal spin conductors, normal spin conductors, and spin insulators.
Assuming diffusive spin dynamics, we show that the spin diffusion coefficient
is proportional to lim_{omega rightarrow 0} {Re} {G}_s (omega). We exploit this
fact to develop a new extrapolation scheme for the spin diffusion coefficient
in the paramagnetic phase of the Hubbard model.
