Dynamic susceptibility curves observed as a function of frequency often deviate considerably from curves predicted by the simple Casimir-Du Pré model. In the case of cerium magnesium nitrate these deviations are mainly caused by the poor thermal conductivity of the liquid or gaseous helium that surrounds the sample and becomes extremely large when the external magnetic field becomes larger than approximately 1 T. We found that, even in this extreme situation, the influence of the environment can be accurately described with the thermal conduction model developed by Flokstra et al. We also discovered that the lattice temperature is stabilized much more effectively if the sample is placed in helium gas at its saturation pressure. The thin helium film that is formed on the surface of the sample provides, through condensation and reevaporation, a far better thermal stabilization of the lattice than bulk liquid helium does. The susceptibility curves that are observed under these conditions differ much less from the Casimir-Du Pré curves.