I've bought a few cars over the years, mostly new from dealers.
There are 2 steps here:
1) Work out which car you want and if it's mechanically OK; and
2) Negotiate the price.
In terms of (1), having someone who knows about cars (NRMA are good) really helps. Having warranty on the car is good too, but factory warranty is really the only warranty that's really worth anything.
For (2), I generally use the direct approach: "I'm happy to buy this car today, but the price needs to be right". Each time I'm asked a question (eg cash or finance, trade or no trade, etc), I simply respond with "which option makes it easier for you to give e a better price"? I never buy dealer options such as paint protection, tinting, upspec wheels, etc, as these are heavily loaded with dealer profit. Also, I will often ask "do you have anything similar to this car in the warehouse that you need to get rid of"?
If you are happy with more than one colour, can consider more than one spec level, and can take a vehicle that's been in the warehouse a while, you can get a very substantial discount. I got about 20% off my V8 when new, back in 2003, because it was in the warehouse, and was a colour/engine combination that wasn't popular (ie boring colour, no bodykit, V8 engine that older folk didn't want). (I still have this car, and still love it!)
To make this work, you need to know what local dealers are buying and selling these cars for. Do your research, and ask car-loving friends. Like buying property, don't let emotion come into it.