  We consider a system of two bosonic modes each subject to the dynamics
induced by a thermal Markovian environment and we identify instantaneous, local
symplectic controls that minimise the loss of entanglement in the Gaussian
regime. By minimising the decrease of the logarithmic negativity at every
instant in time, it will be shown that a non-trivial, finite amount of local
squeezing helps to counter the effect of decoherence during the evolution. We
also determine optimal control routines in the more restrictive scenario where
the control operations are applied on only one of the two modes. We find that
applying an instantaneous control only at the beginning of the dynamics, i.e.
preparing an appropriate initial state, is the optimal strategy for states with
symmetric correlations and when the dynamics is the same on both modes. More
generally, even in asymmetric cases, the delayed decay of entanglement
resulting from the optimal preparation of the initial state with no further
action turns out to be always very close to the optimised control where
multiple operations are applied during the evolution. Our study extends
directly to mono-symmetric systems of any number of modes, i.e. to systems that
are invariant under any local permutation of the modes within any one
partition, as they are locally equivalent to two-mode systems.
