Well, not quite certain how similar things are in Oz, but my suggestions are:
1. Go with a smaller lender (e.g., a credit union) over a large bank -- they tend to offer better rates.
2. Secure financing BEFORE you go.
3. Do NOT let the dealership know how much financing you have -- just tell them you have it and strike the best deal you can.
4. Do NOT take the first offer they throw at you -- I've stormed out of dealerships angrily, and guess what, they generally call back tomorrow with a counter offer (maybe this is a USA thing
)
5. Do not bother getting any extra bells and whistles the dealer offers (e.g., undercarriage coating, etc.) it's never worth it. Even extended warranties aren't worth it -- they can be gotten at a later time, and unless you plan on keeping the car for longer than 6 years, why bother?
6. The dealer may offer to get you a better rate -- that's dubious and allowing them to check will ding your credit -- just stick with what you have, it's probably about as good as its going to get.