Lea-chan has brought up a good point: in a store, the price is usually fixed, so asking for a discount on a single item makes you look very rude. However, if you're doing business with a supplier then it's acceptable. If you're buying in bulk, or buying a number of items, then there's a tiny possibility.
If you want to negotiate, then do it in the most diplomatic way possible. "Would any discount be possible on that?" "Is it possible for you to give a discount?"
In most cases, don't be surprised if the answer is "no". It's a cultural difference.