  A simple reweighting scheme is proposed for Monte Carlo simulations of
interacting particle systems, permitting one to study various parameter values
in a single study, and improving efficiency by an order of magnitude. Unlike
earlier reweighting schemes, the present approach does not require knowledge of
the stationary probability distribution, and so is applicable out of
equilibrium. The method is applied to the contact process in two and three
dimensions, yielding the critical parameter and spreading exponents to
unprecedented precision.
