Let S_k be the set of separable states on B(C^m \otimes C^n) admitting a
representation as a convex combination of k pure product states, or fewer. If
m>1, n> 1, and k \le max(m,n), we show that S_k admits a subset V_k such that
V_k is dense and open in S_k, and such that each state in V_k has a unique
decomposition as a convex combination of pure product states, and we describe
all possible convex decompositions for a set of separable states that properly
contains V_k. In both cases we describe the associated faces of the space of
separable states, which in the first case are simplexes, and in the second case
are direct convex sums of faces that are isomorphic to state spaces of full
matrix algebras. As an application of these results, we characterize all affine
automorphisms of the convex set of separable states, and all automorphisms of
the state space of B(C^m otimes C^n). that preserve entanglement and
separability.