  Using the recent experimental data of $B\to D^{(*)}(\pi,\rho)$, $B\to D^{(*)}
D_s^{(*)}$, $B\to J/\psi K^{(*)}$ and various model calculations on form
factors, we re-analyze the effective coefficients a_1 and a_2 and their ratio.
QCD and electroweak penguin corrections to a_1 from $B\to D^{(*)}D_s^{(*)}$ and
a_2 from $B\to J/\psi K^{(*)}$ are estimated. In addition to the
model-dependent determination, the effective coefficient a_1 is also extracted
in a model-independent way as the decay modes $B\to D^{(*)}h$ are related by
factorization to the measured semileptonic distribution of $B\to D^{(*)}\ell
\bar\nu$ at $q^2=m_h^2$. Moreover, this enables us to extract model-independent
heavy-to-heavy form factors, for example,
$F_0^{BD}(m_\pi^2)=0.66\pm0.06\pm0.05$ and
$A_0^{BD^*}(m_\pi^2)=0.56\pm0.03\pm0.04$. The determination of the magnitude of
a_2 from $B\to J/\psi K^{(*)}$ depends on the form factors $F_1^{BK}$,
$A_{1,2}^{BK^*}$ and $V^{BK^*}$ at $q^2=m^2_{J/\psi}$. By requiring that a_2 be
process insensitive (i.e., the value of a_2 extracted from $J/\psi K$ and
$J/\psi K^*$ states should be similar), as implied by the factorization
hypothesis, we find that $B\to K^{(*)}$ form factors are severely constrained;
they respect the relation $F_1^{BK}(m^2_{J/\psi})\approx 1.9
A_1^{BK^*}(m^2_{J/\psi})$. Form factors $A_2^{BK^*}$ and $V^{BK^*}$ at
$q^2=m^2_{J/\psi}$ inferred from the measurements of the longitudinal
polarization fraction and the P-wave component in $B\to J/\psi K^*$ are
obtained. A stringent upper limit on a_2 is derived from the current bound on
$\ov B^0\to D^0\pi^0$ and it is sensitive to final-state interactions.
