  We present a method for parametrizing heavy meson semileptonic form factors
using dispersion relations, and from it produce a two-parameter description of
the B -> B elastic form factor. We use heavy quark symmetry to relate this
function to B->D* l \nu form factors, and extract
|V_{cb}|=0.037^{+0.003}_{-0.002} from experimental data with a least squares
fit. Our method eliminates model-dependent uncertainties inherent in choosing a
parametrization for the extrapolation of the differential decay rate to
threshold. The method also allows a description of B -> D l nu form factors
accurate to 1% in terms of two parameters.
