We study the dynamics of the spin-boson model using a simple mean-fiel
d-type approximation which treats phonons classically. This simple app
roximation allows us to cover the whole range of system parameters and
to determine boundaries between regimes with different behaviours. Th
e validity of the model is tested by its ability to recover most of th
e previous results. The results are also compared with the results of
some more sophisticated variational approaches.