We study the exclusive rare decay B -> K l(bar) l. We calculate the relevant
form factors within a relativistic constituent quark model, for the first time
without employing the impulse approximation. The calculated form factors are
used to evaluate differential decay rates and polarization observables. We
present results on the q2-dependence of a set of observables with and without
long-distance contributions. A similar analysis is done for the exclusive rare
decays B_c -> D (D*) l(bar) l with special emphasis on the cascade decay B_c ->
D* (-> D pi) l(bar) l. We derive a four-fold angular decay distribution for
this process in terms of helicity amplitudes including lepton mass effects. The
four-fold angular decay distribution allows to define a number of physical
observables which are amenable to measurement. We compare our results with the
results of other studies.Comment: 28 pages, 12 figure
