Indirect searches with gamma rays set strong constraints on the properties of self-annihilating dark matter particle candidates. Observations of dwarf galaxies and galaxy clusters are especially interesting. Here we revisit the calculation of the annihilation flux by considering a velocity-dependent cross-section (Sommerfeld enhanced, for both $s$-wave and $p$-wave) and the boost factor due to dark matter substructure. We show that the combination of these two effects can lead to dramatic enhancements of the gamma-ray flux, and possibly modify the hierarchy between the brightest targets in the sky.