  We formulate an effective model for B-B' site ordering in double perovskite
materials A_2BB'O_6. Even within the simple framework of lattice-gas type
models, we are able to address several experimentally observed issues including
nonmonotonic dependence of the degree of order on annealing temperature, and
the rapid decrease of order upon overdoping with either B or B' species. We
also study ordering in the `ternary' compounds A_2BB'_{1-y}B''_yO_6 Although
our emphasis is on the double perovskites, our results are easily generalizable
to a wide variety of binary and ternary alloys.
