Less air in your tires.
Wait, that's for mtb'ing.
You're down to springs, shocks/struts. Visit the handling section and read up, everything you will want to know about lowering your car and fixing your ride is found there. Of course it's going to cost some $$...so it's either that or going back to stock wheels/tires.
2011 Black on black GT/CS A6
BMR springs, LCA's, panhard, STR.Ts, GT500 AB.