We just start looking and try various date combinations to see what impact it has on the price.
If you're happy to vary the start / end date you can sometimes save a few dollars.
We often go to LastMinute.com.au for our flights.
Once we have a price, then go and check with the majors eg Qantas, Emirates, BA, Lufthansa (OK - we go to europe but same principal)
Be careful to make sure that there is enough time for transit in connecting flights, particularly through US airports.