I might have given up on being a programmer entirely if not for pair programming! Although I didn't do it on-the-job, I made the fastest strides towards good practices and cleaner code when pairing. I think it's probably best to do this only once or twice a week for a max of an hour, and only on parts they need help with. Doing code reviews during this time will help them to improve as well. Always having someone on their shoulder isn't going to help over time, but rather cause conflict.