Haggle, Haggle, Haggle. Your the one with the money and the dealer would like to get his grubby hands on it!
Never ever pay list price and go for at least 10% off, as a starting point. Be prepared to stand up and start walking away. The sight of you (they see you as £notes) walking out the door makes them think again. Bargain for everything including floormats, full tank of fuel.
If your buying secondhand take someone knowledgeable with you to inspect the car, who can shake their head at every minor imperfection. If you cannot take anyone ask the dealer if he minds an AA inspection.
Rather than new have you thought of nearly new? That's how I bought my last car at secondhand prices plus a discount, plus a couple of bibs and bobs thrown in. I retained the manufacturers warranty and free breakdown cover as well. The car was 3mths old with 35miles on the clock and had been in a showroom all the time.