  We present a study of the $D$ and $B$ leptonic decay constants on the MILC
$N_f=2+1$ asqtad gauge ensembles using asqtad-improved staggered light quarks
and clover heavy quarks in the Fermilab interpretation. Our previous analysis
\cite{Bazavov:2011aa} computed the decay constants at lattice spacings $a
\approx 0.14, 0.11$ and $0.083$ fm. We have extended the simulations to finer
$a \approx 0.058$ and $0.043$ fm lattice spacings, and have also increased
statistics; this allows us to address many important sources of uncertainty.
Technical advances include a two-step two-point fit procedure, better tuning of
the heavy quark masses and a better determination of the axial-vector current
matching. The present analysis remains blinded, so here we focus on the
improvements and their predicted impact on the error budget compared to the
prior analysis.
