  We describe in detail a simple and efficient Green Function Monte Carlo
technique for computing both the ground state energy and the ground state
properties by the ``forward walking'' scheme. The simplicity of our
reconfiguration process, used to maintain the walker population constant,
allows to control any source of systematic error in a rigorous and systematic
way. We apply this method to the Heisenberg model and obtain accurate and
reliable estimates of the ground state energy, the order parameter and the
static spin structure factor S(q) for several momenta. For the latter quantity
we also find very good agreement with available experimental data on the
La2CuO4 antiferromagnet.
