for example to balance the equation:
AlCl3 + Na2SO4 ---> Al2(SO4)3 (ppt) + NaCl
Balance the polyvalent atoms (Al, SO4) first. Then everything else falls into line. You need two Al's on the reactants side, but you also need 3 sulfates, so:
2AlCl3 + 3Na2SO4 ---> Al2(SO4)3 + 6NaCl