How to be persistent in a polite way?
Having worked in sales and academia, I have often come across the "how often do I contact this person" question. The answer heavily depends on the situation and the person whom you wish to contact.

You have contacted him twice thus far, and he encouraged your being persistent. I would think he would have no problem in you checking back with him every two weeks. Going to his office may even be suggested after several attempts had been made.

Try to approach the professor in your emails with courtesy, but also with urgency. Always thank him for his time, but also stress that you would like to get your research plans finalized, so if he could respond by such and such date it would be helpful.