  A Markovian master equation describing the evolution of open quantum systems
in the presence of a time-dependent external field is derived within the
Bloch-Redfield formalism. It leads to a system--bath interaction which depends
on the control field. Optimal control theory is used to select control fields
which allow accelerated or decelerated system relaxation, or suppression of
relaxation (dissipation) altogether, depending on the dynamics we impose on the
quantum system. The control--dissipation correlation and the non-perturbative
treatment of the control field are essential for reaching this goal. The
optimal control problem is formulated within Pontryagin's minimum principle and
the resulting optimal differential system is solved numerically. As an
application, we study the dynamics of a spin-boson model in the strong coupling
regime under the influence of an external control field. We show how trapping
the system in unstable quantum states and transfer of population can be
achieved by optimized control of the dissipative quantum system. We also used
optimal control theory to find the driving field that generates the quantum
Z-gate. In several cases studied, we find that the selected optimal field which
reduces the purity loss significatively is a multi--component low--frequency
field including higher harmonics, all of which lie below the phonon cutoff
frequency. Finally, in the undriven case we present an analytic result for the
Lamb shift at zero temperature.
