I did it when i bought my house, walked into currys and bought a 50" plasma and a ps3 bundle.

I could have bought the lot about £80 less online than I could have in currys, so when he totalled it up on the till, I said I'll give you £200 less than that (cant remember what the figure is now).

His face was utter disbelief, as if I was being stupid. He said we can take £30 off for you sir! I said no, I will pay £120 less than the sale price, and if not then I will walk away and buy it elsewhere online.

Needless to say I left a happy man