I too was clueless about centering on the wheel, after two
college classes, and no luck, I
finally took a one day kid's workshop and the teacher said,
"Hands don't move!"
"When your hands are still, the clay is centered."
I had been trying to "feel the clay centered". Anyway,
hope this helps some of you.
Other thing was, I needed to level my portable wheel each
time I moved it! A slight slant ruins the symmetry.