  A new approach to dissipative quantum systems modelled by a system plus
environment Hamiltonian is presented. Using a continuous sequence of
infinitesimal unitary transformations the small quantum system is decoupled
from its thermodynamically large environment. Dissipation enters through the
observation that system observables generically decay completely into a
different structure when the Hamiltonian is transformed into diagonal form. The
method is particularly suited for studying low-temperature properties. This is
demonstrated explicitly for the super-Ohmic spin-boson model.
