A good practice for balancing equations with polyatomic ions is:

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