  We propose a new method to determine the unknown parameter associated to a
self-consistent harmonic approximation. We check the validity of our technique
in the context of the sine-Gordon model. As a non trivial application we
consider the scaling regime of the 2D Ising model away from the critical point
and in the presence of a magnetic field $h$. We derive an expression that
relates the approximate correlation length $\xi$, $T-T_c$ and $h$.
