It's difficult to get a visa in the first place, so I assume extending one is even more difficult. What kind of visa do you mean? Certain types can not be extended, but I think the tourist visa may be the easiest out of the ones that can.

A working visa? What's the problem? The company who sponsored you and for whom you work handles it. Never a problem.

I think it was very easy for me as brazilian to get all the visas, beureocratic though

Vica 2008:

you do not say which type of visa is difficult to extend, so my answer can not be specific. 

In some instances, requirements to get a visa extension (on the number of days to stay, not the validity) far exceed what was required to issue the visa.  You can apply for a tourist visa without even showing a bank account, but try to get a 30 day extension on your stay, and need to show a local bank savings book with at least 30,000 Rmb deposited in it among other things.  On a work visa, your employer should do all the leg work, not you, so what is the difficulty?.   With a student visa, as long as you remain registered at a school or college you get another year.  There are other types of visa, like transit, journalist (actually two types of this one), diplomatic, etc, but those are not our concern.

Unless you are more specific in your question, I really do not know what to answer.