Improved harmonic approximation and the 2D Ising model at T &not=; Tc and h &not=; 0

We propose a new method to determine the unknown parameter associated with 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 ξ, T - T_c and h.