Read more and write more, reading is very essential, you can't simply improve your writing just by writing. You should read a lot of good and classical books to learn advanced words and sentence structures.

my experience is:

firstly, you read...

especially some master piece of some great witters/authors in the human history of the world...

to see how those great guys make words dance under their pens...

and in the meantime, you'd also like to read some good pieces of the contemporary styles, in order to keep up with the time...

meanwhile, you practice and imitate and think and accumulate to try and develop your own style...