Welcome AussieBattler. The best advice I can give for maximising the DONE4 is as follows:

Look for AA 2-class flights over 2700 miles. This gets you seated in F (really more like J anyway), but also scores you 150 SC's each way as it is an F flight. Some examples are ORD-ANC, DFW-ANC (these are seasonal in summer only), and SFO-BOS. Since ORD is in your plane, why not take an overnight scoot up to ANC and score 300 SC's along the way.

Also, make sure you break the journey in SIN on the way back. Don't just take the one flight number all the way through. You get 220 SC's for LHR-SYD and 160+100SC for LHR-SIN-SYD. But it must be two coupons in your ticket book.

In Europe, LHR-HEL is over 1000 miles, so a good side trip also.

If you want to take the long way home, LHR-HKG-NRT-MEL-PER-SYD is always a good run at 160+60+160+60+60=500 SC's.

There are a few very good threads over in the OneWorld board explaining how to maximise these routings.