A system of equations of motion for the Green functions in layered antiferromagnets such as La2CuO4 and YBa2Cu3O6 has been treated by an “equal access” decoupling scheme. The correlation functions are fully equivalent in this scheme in contrast with the ordinary random-phase approximation (RPA). The method provides a new insight into the nature of the RPA treatment of the localized spin dynamics in magnets. Explicit self-consistent expression for the sublattice magnetization in a perpendicular field is given. The dependence of the sublattice magnetization on a perpendicular magnetic field is studied. High temperature tails of the in-plane sublattice magnetization have been found to result from a nonlinear coupling of the perpendicular magnetic field with the antiferromagnetic order parameter.