Land Transport

Use alternatives to the car where practicable (walk, bike, skate).

Public transport
Trip planners for Sydney, Melbourne, Perth, Brisbane, Adelaide, Hobart, Canberra

Car Pool
http://www.carpoolaustralia.com/ free registration
www.thecarpool.com.au for school and car pools (free registration)
www.needaride.com.au for long distance travel

Flexicar
www.flexicar.com.au hire an economical car by the hour!
An envioronmentally friendly alternative to owning a car.

How economical is your car?
If you need to drive then drive an economical car.
Check www.greenvehicleguide.gov.au for ratings
or www.greenercars.org/highlights_greenest.htm

Passenger Vehicles
Regular maintenance – check tyre pressure, wheel alignment, engine timing, air filter and spark plugs. If the tyre pressure is too low, 5% of your fuel could be wasted. Slow down – as well as increasing accident risks, driving over the speed limit on the open road increases fuel costs by up to 10%.
Drive smoothly and look ahead – maintain a safe following distance and you’ll keep at a more constant speed. This uses less fuel than accelerating and braking all the time. Watch the air con – having the air-conditioning on uses up to 10% more fuel. Keep the load down and reduce fuel consumption by up to 5%. Source

Fleet Vehicles
Lots of useful tips from the Envrionmental Defence Fund at http://innovation.edf.org/page.cfm?tagID=27202&redirect=greenfleet 

Commercial Vehicles
There are many techniques for optimising fuel efficiency. Ask the manufacturer about adding hydrogen to the air intake, high octane fuels, fuel additives and regular engine maintenance. Other easy techniques are weight minimisation, fuel economy metrics, efficient route planning and two way trips (collecting when delivering). UPS in America has been using hybrid delivery vehicles for some years and is expanding their fleet manyfold.
Keeping tyres optimally inflated uses less fuel, as does reducing weight being lighter on the pedal and keeping the engine in tune.

Air Transport

Air travel is hugely emission intensive, especially for short flights. The best option is not to fly if you can help it.
If you must fly then you can estimate and offset your flights emissions, here are some useful sites;
http://www.carbonbalanceconsulting.com.au/flights_calculator Simple carbon calculator
www.chooseclimate.org High level of detail for exact calculations

Carbon Footprint Calculators

http://www.1degree.com.au/carbon_calculator
http://www.wwf.org.au/footprint/calculator/
www.epa.vic.gov.au/ecologicalfootprint/calculators/personal/completed.asp

Carbon Offsets

Carbon offsets, like green power, come in all shades of green. Checkout Carbon Offset Watch for rankings.
ARK Resources www.ark-resources.com.au
Low Energy Supplies and Services (LESS) www.lowenergy.com.au
Carbon Planet www.carbonplanet.com
Easy Being Green http://www.easybeinggreen.com.au/Green_tips.htm
Climate Friendly www.climatefriendly.com