Originally Posted by JohnDThompson

If you cannot eliminate play without tightening the headset so much that it no longer freely turns, there are a couple things to check: first, make sure the bearings are installed properly. If a retainer or cartridge is installed upside down it can cause adjustment problems. Second, the faces of the head tube may need to be prepped to ensure that the upper and lower races are both installed concentrically and perpendicular to the axis of the head tube. If this is not the case, the headset will appear to be tight in one spot, but loose in another. And don't forget to check that the crown race is fully seated, as hueyhoolihan notes above.