  Following the strategy developed by the ALPHA collaboration, we present a
method to compute non-perturbatively the decay constant of a heavy-light meson
in HQET including the 1/m corrections. We start by a matching between HQET and
QCD in a small volume to determine the parameters of the effective theory
non-perturbatively. Observables in the effective theory are then evolved to
larger volumes. In two steps a large enough volume is reached to determine the
physical decay constant. Some preliminary results in the quenched approximation
are shown.
