  We discuss a general and systematic method for obtaining effective
Hamiltonians that describe different nonlinear optical processes. The method
exploits the existence of a nonlinear deformation of the usual su(2) algebra
that arises as the dynamical symmetry of the original model. When some physical
parameter, dictated by the process under consideration, becomes small, we
immediately get a diagonal effective Hamiltonian that correctly represents the
dynamics for arbitrary states and long times. We extend the technique to su(3)
and su(N), finding the corresponding effective Hamiltonians when some resonance
conditions are fulfilled.
