A) Don't be ridiculous with your price
B) There is no such as "sorry, i can't do that for you"
C) Be FIRM, but FAIR.
D) Go in with the attitude that you wouldn't care if you left the dealership without the car.
E) If you're getting close to a deal, try and get every. little. thing. thrown in. Ie. New matts, if there is a scratch on the car, get that repaired, if the alloys are kerbed, ask to have them refurbed. Ask them to throw in tax (a lot of dealerships will say "sorry, we can't tax cars" but THIS IS RUBBISH). Push for things, but don't overstretch.
Lots of good lines include "Right, we're getting close to a deal, you're not going to let it fall through over 6 months tax are you??" and holding out your hand to shake when you think you're close enough to a deal.
Obviously you can't go in there and say "I'll offer 6k" and hold out your hand because they'll throw you out

, but if you've got down to the little extras like mats and tax it's an awesome way of getting what you want.