Bloch oscillations of atoms in an optical lattice are a technique commonly used to accelerate atoms using photon recoils. The initial atomic state is crucial for optimizing the efficiency of the acceleration. Usually, a Bloch state is prepared by adiabatically loading atoms into the lattice. In this paper, using numerical simulations, we show that the optimal state (the Wannier-Bloch state) can significantly differ from the Bloch state. Furthermore, two experimental methods are presented in order to get closer to the optimal efficiency.