  We present a general interpolation theory for the phenomenological effects of
thermal fluctuations in superconductors. Fluctuations are described by a simple
gauge invariant extension of the gaussian effective potential for the
Ginzburg-Landau static model. The approach is shown to be a genuine variational
method, and to be stationary for infinitesimal gauge variations around the
Landau gauge. Correlation and penetration lengths are shown to depart from the
mean field behaviour in a more or less wide range of temperature below the
critical regime, depending on the class of material considered. The method is
quite general and yields a very good interpolation of the experimental data for
very different materials.
