Compare your options online. The easiest way to find a better deal is to see all the prices from different companies for your dates in one place.
Book ahead during industrial shutdowns. Gladstone is a working town. When major projects or maintenance are on, demand for cars skyrockets, and so do prices.
Do the math on your pickup spot. An airport pickup can have extra fees, but getting to a city depot will cost you a taxi fare. Weigh the total cost to see which is actually cheaper.
Check the weekly rate. If you need a car for 5 or 6 days, it's worth checking the price for a full 7-day rental. Sometimes the weekly deal is cheaper.