How to choose which reactants to have in excess? In a precipitation reaction, how do I know which soluble reactant I should put in excess?
E.G. 
$$\ce{Na2SO4 +Pb(NO3)2 -&gt; PbSO4 + 2NaNO3}$$
In my book, $\ce{Na2SO4}$ is added in excess, but can we add $\ce{Pb(NO3)2}$ in excess?
Sometimes the excess is the cheap/unlimited reagent you use to get a high yield on your expensive/limited reagent, and sometimes you use the one that you don't mind/can easily extract from the products.

In this case, if you wanted to collect and record the lead(II) sulphate as a precipitate, using an excess of the lead(II) nitrate would be a waste.  If your intention was to produce a large quantity of sodium nitrate solution, and you don't mind having a load of the lead nitrate contaminating it, then you could use an excess of the lead nitrate.