  We consider a single copy of a mixed state of two qubits and derive the
optimal trace-preserving local operations assisted by classical communication
(LOCC) such as to maximize the fidelity of teleportation that can be achieved
with this state. These optimal local operations turn out to be implementable by
one-way communication, and always yields a teleportation fidelity larger than
2/3 if the original state is entangled. This maximal achievable fidelity is an
entanglement measure and turns out to quantify the minimal amount of mixing
required to destroy the entanglement in a quantum state.
