1) go to the Chinese embassy/consulate and re-apply for a visa with a longer period of stay in China.
2) go to China and when the visa is about to expire, go to the Office of CHU RU JING GUAN LI JU in Beijing and extend your stay in China
3) it is also possible to extend your visa at the Beijing international airport just before you clear the Chinese customs (there is a booth). You need to check this one out just to make sure...