Variational approaches for nonlinear elasticity show that Hill's incremental formulation for the prediction of the overall behaviour of heterogeneous materials yields estimates which are too sti and may even violate rigorous bounds. This paper aims at proposing an alternative `a ne' formulation, based on a linear thermoelastic comparison medium, which could yield softer estimates. It is ®rst described for nonlinear elasticity and speci®ed by making use of Hashin±Shtrikman estimates for the linear comparison composite; the associated a ne self-consistent predictions are satisfactorily compared with incremental and tangent ones for power-law creeping polycrystals. Comparison is then made with the second-order procedure (Ponte CastanÄ eda, P., 1996. Exact second-order estimates for the e ective mechanical properties of nonlinear composite materials. J. Mech. Phys. Solids, 44 (6), 827±862) and some limitations of the a ne method are pointed out; explicit comparisons between di erent procedures are performed for isotropic, two-phase materials. Finally, the a ne formulation is extended to history-dependent behaviours; application to the self-consistent modelling of the elastoplastic behaviour of polycrystals shows that it o ers an improved alternative to Hill's incremental formulation.