  This paper reports a detailed study of techniques for identifying boosted,
hadronically decaying $W$ bosons using 20fb$^{-1}$ of proton-proton collision
data collected by the ATLAS detector at the LHC at a centre-of-mass energy
$\sqrt{s} =$ 8 TeV. A range of techniques for optimising the signal jet mass
resolution are combined with various jet substructure variables. The results of
these studies in Monte Carlo simulations show that a simple pairwise
combination of groomed jet mass and one substructure variable can provide a 50%
efficiency for identifying $W$ bosons with transverse momenta larger than 200
GeV while maintaining multijet background efficiencies of 2-4% for jets with
the same transverse momentum. These signal and background efficiencies are
confirmed in data for a selection of tagging techniques.
