  We present a simple and efficient method to set up spherical structure models
for N-body simulations with a multimass technique. This technique reduces by a
substantial factor the computer run time needed in order to resolve a given
scale as compared to single-mass models. It therefore allows to resolve smaller
scales in N-body simulations for a given computer run time. Here, we present
several models with an effective resolution of up to 1.68 x 10^9 particles
within their virial radius which are stable over cosmologically relevant
time-scales. As an application, we confirm the theoretical prediction by Dehnen
(2005) that in mergers of collisonless structures like dark matter haloes
always the cusp of the steepest progenitor is preserved. We model each merger
progenitor with an effective number of particles of approximately 10^8
particles. We also find that in a core-core merger the central density
approximately doubles whereas in the cusp-cusp case the central density only
increases by approximately 50%. This may suggest that the central region of
flat structures are better protected and get less energy input through the
merger process.
