We have some black scuff marks on the bottom of our kitchen chairs, which are wooden. We have cleaned these a number of times and the scuffs keep returning. Any ideas on how to remove these marks forever?

Thanks for the help!
Ok, lets get away from wearing our shoes in the kitchen area. That is not going to happen. What I need is an answer that is going to tell me how to keep the bottom of my chairs clean in the future. Can I put on anything on them to protect them etc… thanks again for all the suggestions!