  Numerical renormalization group (NRG) calculations of quantum impurity
models, based on a logarithmic discretization in energy of electronic or
bosonic Hamiltonians, provide a powerful tool to describe physics involving
widely separated energy scales, as typically encountered in nanostructures and
strongly correlated materials. This main advantage of the NRG was however
considered a drawback for resolving sharp spectral features at finite energy,
such as dissipative atomic peaks. Surprisingly, we find a bunching of many-body
levels in NRG spectra near dissipative resonances, and exploit this by
combining the widely-used Oliveira's $z$-trick, using an averaging over {\it
few} discrete NRG spectra, with an optimized {\it frequency-dependent}
broadening parameter $b(\w)$. This strategy offers a tremendous gain in
computational power and extracts all the needed information from the raw NRG
data without {\it a priori} knowledge of the various energy scales at play. As
an application we investigate with high precision the crossover from coherent
to incoherent dynamics in the spin boson model.
