Go to sites like RACV and Carsales/carpoint... Have a look at the particular car you are going in to buy. Circle some stuff... Let the seller know that you KNOW how much the cars are really going for... Let them know that if you dont buy the car off them you are more than happy to go and buy off someone else... Show them that you've done a bit of research into it.
For everything good they say about it point out something negative (eg. "and it has a powerful V8", you say "Hmmm, I bet that goes through a bit of petrol")
Dont seem too excited, play it cool... Make sure the seller knows whos in control (you!)
All that being said... Dont go too far with it, if you know its a good price just try and haggle down, don't be too aggressive.
Also... At the end of it, ask for a full tank of petrol to be thrown in, I did, even get the guy to write it in the contract) $70 bucks better in your pocket than there's!. Do that when you cant haggle them down any further and your happy with the price.
Good luck to ya mate!! It's a nerve racking and fun experience buying your first car!