Besides collecting shopping dockets to combat petrol price hikes, I guess another way is to join free-to-enter competitions that can actually win petrol. Two sites that I have found:

• Drive.com.au — win $100 worth of petrol 3 times a day. I have been entering my details a few times a week ever since it has started, but have not won anything :( But at least if you click through via OzRewardS, you get 5 cents rebated into your account, and they do add up…
• CARguide.news.com.au — win $300 worth of petrol on 5^th of August, and there will be 10 winners.

Alternatively, you can always take public transport, buy a more fuel efficient car, or get a job that allows you to do telecommuting…