  We present an alternative way to determine the unknown parameter associated
to a gaussian approximation in a generic two-dimensional model. Instead of the
standard variational approach, we propose a procedure based on a quantitative
prediction of conformal invariance, valid for systems in the scaling regime,
away from criticality. We illustrate our idea by considering, as an example,
the sine-Gordon model. Our method gives a good approximation for the soliton
mass as function of $\beta$.
