Edit: few things:
* Most important of all - don't be a dick. Be friendly, honest and don't waste people's time
* If you can, hold of your purchase until you get the deal you're looking for. When I got my macbook, I when to a number of places and in the end got free applecare and a free case.
* If you're buying white/brown goods (except computers), a simple "What price can you do on that?" will almost always see a 10% discount, sometimes way more.
* Some stock is in constant demand, and you'll almost never get a discount. Black mens trousers, for example.
* If something on offer, the person selling it want's to get rid of it, and will most likely negotiate. Best example is food at the end of the day. Walking past a stand discounting the remaining donuts for $6/box, but you've only got $4? Make an offer anyway.
er... think I'l stop before this looks like a post on Lifehacker or something :)