I'm a bit anal about the centers in my shafts, and basically I center them twice. The first part is as described by others above, using grain alignment, then a center drill held freehand in a cordless drill. However, to ensure that the centers are perfectly perpendicular to the axis of the shaft for full contact when driving them during later cutting, I then take a straight pass on the piece held between the "rough" centers that I just installed. Now that the outside of the piece is perfectly round and concentric to the "rough" centers, I hold each end in the lathe chuck and re-center lightly, this time with the center drill held in the tailstock. Now both centers are in perfect alignment with each other for full contact when driving on a saw type shaft machine, or any other shaft machine for that matter.