Threaten to cancel the order
When I ordered a Dell XPS M1330 over a year ago, the delivery date was pushed back three times. So I phoned up to cancel the order, I was told by the CSR that the LED backlit display panels were out of stock, I insisted that I wanted to cancel the order, then the agent offered to put a 'priority' on my order and have it shipped the next working day. Sure enough, it arrived the following Tuesday morning.