You should know only two sentences:
Duoshao tian? How much does it cost?
Pianyi Yidian! Make it cheaper!
A few more tips:
- Check the same product from several shops. In the end you will know the product price.
- Don't ask directly the price of the product you want! First start asking the price for one or two products you don't want to buy. Then ask the price of the product you really want!
- After you bargain and asked several time: "pianyi yidian", go away to come back later.
- Buy everything from one same shop. Thus you can ask for more discount!
- When buying several products, first bargain for each product, then bargain for the whole final price.