I made a few pepper and salt mills for christmas gifts, yet I am having a hard time getting perfect concentricity of the head and body. Per the instructions:

Turn rough blank, cut tenon, part the two pieces, them mount the head by the tenon and drill a hole from tailstock into head. Then I mount the body, cut recess in bottom, hollow it halfway, turn around and remount, then cut other half. But when I mount the body to cut the through holes, there is no centermark on the second cut where the tenon fits, so that is off just a hair, but that error doubles when you mount the head and turn it when it is all finished.

Is there any trick to keep those truly concentric?

This is not a problem if you have a design part there in the overall shape, but for a continuous curve style where you want a smooth line between the two pieces, this is an issue. I can get it close, but I want it closer.