I didn't take out finance and still managed to get £8k off on a cash deal. I just told them what i was willing to pay and stuck to it. Walk away and wait for them to chase you. When they rang me I just said my offer still stands you can take a deposit from my debit card now, but they declined. I informed them that was all the money I had for the car and required the spec I had chosen if it was to go ahead. 10 minutes later they rang back for my card details
The other thing you can do is ask if they have any second hand. If they have say you are not keen on the spec. Then ask them as a matter of interest what it would cost to get you in a new one. They will try harder on price to sell new rather than second hand.
Never sound too keen and work out your spec before you even visit the dealer.