Lifehacker’s 2011 Australian Airport Public Transport Guide
Lifehacker has got a great guide to getting to your major capitol city's airport. I can only speak for Melbourne and there's a couple of things that should be added.
Never tried taking the public bus. Anyone every tried this?
Taxi- Usually around $50 from the city, so if you have 2+ people it's probably cheaper doing this.
Airport Shuttle-These are quite big in the US. Have taken SuperShuttle at a number of US airports. Googling around leads me to http://www.vhaairportshuttle.com.au/. $16pp pickup from city and nearby suburbs.
Can't remember where I heard this tip but if you want to short term park for the day, you can try parking in the McDonald's parking lot.