  We present a lattice investigation of the heavy-light meson decay constants
using Wilson light quarks and NRQCD heavy quarks, partially including the
effects of dynamical sea quarks. We calculate the pseudoscalar and vector decay
constants over a wide range in heavy quark mass and are able to perform a
detailed analysis of heavy quark symmetry. We find consistency between the
extrapolation of the NRQCD results and the static case, as expected. We find
the slope of the decay constants with $1/M$ is significantly larger than naive
expectations and the results of previous lattice calculations. For the first
time we extract the non-perturbative coefficients of the slope arising from the
$O(1/M)$ heavy quark interactions separately and show the kinetic energy of the
heavy quark is dominant and responsible for the large slope. In addition, we
find that significant systematic errors remain in the decay constant extracted
around the $B$ meson mass due to truncating the NRQCD series at $O(1/M)$. We
estimate the higher order contributions to $f_B$ are approximately $20\%$;
roughly the same size as the systematic errors introduced by using the Wilson
action for light quarks.
