  We present results for different observables in weak decays of pseudoscalar
and vector mesons with a heavy $c$ or $b$ quark. The calculations are done in a
nonrelativistic constituent quark model improved at some instances by heavy
quark effective theory constraints. We determine pseudoscalar and vector meson
decay constants that within a few per cent satisfy $f_V M_V/f_P M_P=1$, a
result expected in heavy quark symmetry when the heavy quark masses tend to
infinity. We also analyze the semileptonic $B\to D$ and $B\to D^*$ decays for
which we evaluate the different form factors. Here we impose heavy quark
effective theory constraints among form factors that are not satisfied by a
direct quark model calculation. The value of the form factors at zero recoil
allows us to determine, by comparison with experimental data, the value of the
$|V_{cb}|$ Cabbibo-Kobayashi-Maskawa matrix element. From the $B\to D$
semileptonic decay we get $|V_{cb}|=0.040\pm0.006$ in perfect agreement with
our previous determination based on the study of the semileptonic $\Lambda_b\to
\Lambda_c$ decay and also in excellent agreement with a recent experimental
determination by the DELPHI Collaboration. We further make use of the partial
conservation of axial current hypothesis to determine the strong coupling
constants $g_{B^*B\pi}(0)=60.5\pm 1.1$ and $g_{D^*D\pi}(0)=22.1\pm0.4$. The
ratio $R=(g_{B^*B\pi}(0) f_{B^*}\sqrt{M_D})/ (g_{D^*D\pi}(0)
f_{D^*}\sqrt{M_B})=1.105\pm0.005$ agrees with the heavy quark symmetry
prediction of 1.
