Coupling of external light signals into a photonic crystal waveguide becomes increasingly inefficient as the group velocity of the waveguiding mode slows down. We have systematically studied the efficiency of coupling in the slow light regime for samples with different truncations of the photonic lattice at the coupling interface. The coupling efficiency is found to be significantly improved up to group indices of 100 for a truncation of the lattice that favors the appearance of the photonic surface states at the coupling interface in resonance with the slow light mode