  Observations of high-redshift supernovae indicate that the universe is
accelerating. Here we present a {\em model-independent} method for estimating
the form of the potential $V(\phi)$ of the scalar field driving this
acceleration, and the associated equation of state $w_\phi$. Our method is
based on a versatile analytical form for the luminosity distance $D_L$,
optimized to fit observed distances to distant supernovae and differentiated to
yield $V(\phi)$ and $w_\phi$. Our results favor $w_\phi\simeq -1$ at the
present epoch, steadily increasing with redshift. A cosmological constant is
consistent with our results.
