We present a theoretical investigation of the local launching of surface plasmon polaritons (SPPs) by means of metal nanostructures. Efficient conversion of a propagating plane wave into interface-bound SPPs is achieved through grooves in the surface of bulk metals or slits in thin metal films. The incident light excites not only SPPs at the metal surface but also the electromagnetic modes inside the groove, which behaves like a cavity so that its width and depth have a strong impact on the SPP excitation efficiency. The light-SPP coupling can furthermore be improved by several grooves interfering constructively. Tuning the width of a slit in a thin metal film allows us to preferentially excite either one of the two fundamental SPP waveguide modes of the system, namely either the mode propagating at the upper metal interface or its counterpart at the lower interface. Maximum conversion efficiencies of more than 50% can be achieved with optimized geometries.