I have been to Hong Kong last month. That was really exciting trip! Especially I am a crazy shopper.... much cheaper in my home town. Some advise for you:

well, i went to Tsim Sha Tsui area where most of the brands located, even Duty Free Shops. Sometimes, you can get discount when you show the passport to prove you are foreigners or visiters.
You can check some information at the Hotel Concierge or email them ahead. They would be able to assist you.

But i found one very interesting website that I can share with you guys, it is www.s-purchaser.com.

I ordered some Hong Kong products, like hand cream, mobile...really good in service. My friend tried this and told me I should share with people who want to go to HK to buy aaaaaaa lot of cheap items, but not able to visit very often.