These were a set of rules devised by Pauling to describe the structures of crystals and are as follows :-
A coordinated polyhedron of anions is formed about each cation, the cation-anion
distance determined by the sum of ionic radii and the coordination
number by the radius ratio.
An ionic structure will be stable to the extent that the sum of the strengths of
the electrostatic bonds that reach an anion equal the charge on that anion.
The sharing of edges and particularly faces by two anion polyhedra decreases the
stability of an ionic structure.
In a crystal containing different cations, those of high valency and small
coordination number tend not to share polyhedron elements with one another.
5.The number of essentially different kinds of constituents in a crystal tends to be small.
Taken from http://en.wikipedia.org/wiki/Pauling%27s_Rules