I scribe the centerline with a hinge centerline marking tool then sand by eye with a nice long aluminum sanding block. If you go slow and have good light, it's not that difficult. Though, on future builds, I may think of some way to make it easier to be more accurate.

EDIT: This is the hinge marking tool I use... http://www3.towerhobbies.com/cgi-bin...?&I=LXK262&P=0
There's other ways to mark a centerline, but this thing makes it so darn easy.