  The flavor-changing neutral current process $b\to s l^+l^-$ is beneficial to
testing the standard model and hunting for new physics scenarios. In exclusive
decay modes like $B\to K^*(892)l^+l^-$, the S-wave effects may not be
negligible and thus have to be reliably estimated. Using the scalar form
factors derived from dispersion relations in two channels and matched to Chiral
Perturbation Theory, we investigate the S-wave contributions in $\bar B^0\to
K^- \pi^+ l^+l^-$, with the $K\pi$ invariant mass lying in the vicinity of the
mass of $K^*(892)$, and the $B_s\to K^- K^+ l^+l^-$ with $m_{KK}\sim m_{\phi}$.
We find that the S-wave will modify differential decay widths by about 10% in
the process of $\bar B^0\to K^- \pi^+ l^+l^-$ and about 5% in $B_s\to K^- K^+
l^+l^-$. A forward-backward asymmetry for the charged kaon in the final state
arises from the interference between the S-wave and P-wave contributions. The
measurement of this asymmetry offers a new way to determine the variation of
the $K\pi$ S-wave phase versus the invariant mass.
