Basketball players practice free throws a lot – and they still miss.  I think centering is a lot like free throws – at first you miss a lot , then you get into a cycle of improving and relapsing.  Finally, you center effectively most of the time.  I say most of the time because I don’t always get my clay centered – usually this is due to bad wedging or dry clay.  But it happens.  For the beginning and intermediate potters, this can be very frustrating.  The key to success is perseverance.  I assure them they will get it with practice.