  A method is proposed to handle the sign problem in the simulation of systems
having indefinite or complex-valued measures. In general, this new approach,
which is based on renormalisation blocking, is shown to yield statistical
errors smaller than the crude Monte Carlo method using absolute values of the
original measures. The improved method is applied to the 2D Ising model with
temperature generalised to take on complex values. It is also adapted to
implement Monte Carlo Renormalisation Group calculations of the magnetic and
thermal critical exponents.
