Cavity-optomechanics is an ideal platform for the generation non-Gaussian quantum states due to the anharmonic interaction between the light field and the mechanical oscillator, but it is exactly this interaction that also impedes the preparation of pure states of the light field. In this paper we derive a driving protocol that helps to exploit the anharmonic interaction for state preparation and that ensures that the state of the light field remains close to pure. This shall enable the deterministic preparation of photon Fock states or coherent superpositions thereof.