Je pense que tu peux lire des journal chinois souvent,surfer sur Internet de la langue chinois etc.si tu as des question,n'hésite pas,je peux t'aider.

listen to the radio,read the newspaper,and talk with Chinese by any way.

你好，Stephan,我并不了解你目前的中文水平。你说你想提高你的读、写普通话的能力，那就是说，你已经会一些中文了，只是想把中文学得更好，是吗？如果我的的理解是正确的，那么，我对你只有一个建议，那就是从现在就开始练习它！
Hello,Stephan, i don't know how's ur Chinese. u said that u want to improve, that means u can writing and reading some Chinese, u just want to do better.If i'm right, i thought i just have one advice to u- Just do now!

那么，也许你可以尽量用中文提问题。
So, maybe u would like to answer ur question in Chinese.

一方面，我们可以通过看你写的中文问题来估计一下你的中文水平，以便提出一些适合你的建议。同时，你也可以练习你的中文写作能力。
We can estimate ur level in Chinese skills by reading ur question sentence, then we can give some suitable advice to u. at the same time, u can practice ur writing skill in Chinese.

不要错过任何练习中文的机会！
Don't lose any chance to practice ur Chinese!
加油！

Just by reading more books. You can choose some people's autobiography or novels which you might have interest. This can not only help you with your reading and writing, but can also acquaint you with Chinese traditional culture.

one way--reading more mandarin articles
and do like that ,you would get waht you want,if you insist
maybe you could feel hard and want to give up,but plz insist for one month,and then all's ok
you can reading and writing in Mandarin now
trust me