How to be creative in doing research? Besides hard work, is there anything that one can try to be creative in research? On this site, is there a lot of people who can publish more than 5 or 10 papers per year? I would like ask them, if it is possible to train oneself to do that.

I have been a hardworking student. For four years, I have tried to publish one (first-author) paper per year, and I feel that this is my limit. This rate is enough for me to get a PhD, but it is never enough for survive in academia.

I'm going to start my postdoc in a couple of weeks, and I'm really worried, thinking about my career. I really love research, but I'm afraid I'm just a mediocre scientist, who will end up as a postdoc for 10 years (or forever).
I would like to suggest that you read the speech "You and Your Research" by Richard Hamming on how to do great research.  It's one that I revisit every once in a while when my own scientific motivations are becoming unclear and need to be refreshed.

In reading it, it's important to keep in mind that the speech is thirty years old, and it is addressing things that happened even decades before that, so there are some jarring cultural gaps both with regards to science and with regards to the rest of society.  Nevertheless, I find the central ideas to be still very relevant and valuable today.

Importantly, Hamming distinguishes between things that you need to do in order to not fail and things that you need to do in order to have a real impact.  These are very different, and it is important to distinguish between them.  Important research is not the same as any of the "productivity metrics," and the correlation is often a trailing indicator, not a leading indicator.

For example: you say you are concerned that you are publishing only one paper per year.  Step back and ask: why am I concerned about this?  You can publish lots of papers without ever doing anything really creative or important.  You can have a full, tenured, and comfortable career that way.  And honestly, that's an OK choice to make.  Some people go too far the other way, and strike out so deep into their own creative world that they can never publish their ideas, and they also fail to have an impact (indeed, the most extreme of them we label cranks).

In between is the dangerous and subversive world that Hamming describes, where you teeter back and forth on the balance between compliant conformity to metrics and egotistical creative rebellion.  It doesn't guarantee greatness, but it may be a fascinating ride, and it just might get you somewhere really worth going.