"It's like not hitting the curb with the front wheel when you park the car, you can't see the wheel, but you get a feeling for where it is."
Crap! I hit the curb all the time
Another technique works quite well for me and it gets better as the planes go up in size. You try to 'put it between your legs'. Simply, instead of trying so hard to keep the midline of the plane directly over the line, try to put the line between your legs. You're close enough to the center of the plane for it to not make a difference. This especially helps when on final approach and you are trying to track the centerline.