How to top-up in china
 

I am going to stay in china for a while, both in shanghai and Beijing.
Could someone advice me on the easiest way to top-up my phone credit. I know there are top-up cards, however I've heard that there are also self service kiosks. How does that work, and do I need any local credit card for that, or cash can also be used? Also which of the mentioned has the cheapest commission?
Thanks in advance!