how can i improve business english??

你可試一試英國文化中心 它們有專門的 English Courses, 包括 Business English 專科 (請查website for details). 但它不是報名便收人, 是要經過考試的. 不知你一般英文程度如何? 試一試, it won't disappoint you.
希望幫到你.

其實冇架
你一定要多聽多講多讀多寫
course可能有用.
但最重要都係睇你冇有心學
如果你有心.即使係dailylife.你都可以學到:)

你所指的business English 是會話或書信？

會話方面可通過閲讀有關書籍或錄音帶學習會話技巧。同時多一點練習，時間久了就能有好成績。

書信方面除了閲讀，還可以嘗試與朋友以英文通信，加強寫作能力。Email不失為一個好辦法！

If you wanna improve business English, I think there should just be one way
out. To have some professional courses.

For my opinion, no matter how hard you try to learn form book or the website. You can't have a result as good as learning from a profession lesson. Therefore, I suggest you not to think about using the books or websites unless you just want to improve the simplest english~

And I must sorry to say that I don't have any suggesed book and website for reference.

you can try
1) SCMP business section. http://www.scmp.com/portal/site/SCMP/
2) The Economist magazine is really good for business English, though you might find it a bit difficult to begin with... http://www.economist.com/