Once your minutes are used up, you can get recharge cards (chongzhika 充值卡) at every kiosk or supermarket around town. The recharge cards come in soft business card sized versions where you have to rub off a seal to get the recharge number or they come in paper versions where you have to cut off the seams for this.
Steps for recharging (for China Mobile)
1) Scratch off the seal to (if the seal is broken, don’t buy the card!)
2) Dial „138 00 138 000“ to reach China Mobile
3) Dial „1“ for Chinese service or „2“ for English (then „wait for a whale“)
4) Dial „2“ for recharge
5) Dial „1“ followed by „#“ („pound or number key“ in English, xinghao in Chinese) to recharge the phone you have in your hand (btw. you could dial a friends number here instead to recharge his phone)
6) Enter the recharge number followed by „#“. If the recharge worked, a voice will tell you your remaining balance.
7) You can disconnect now.