Hi Ali,
The flight is pretty tough going, I go to the states a lot with my work and there are a few rules that I impose on myself for flights:
1- Never watch more than 1 movie per flight, or at least leave a few hours in between to stop the zombification!
2- Don't be afraid to get up, walk around and look silly doing exercises; you need to keep moving or else your legs will puff up ^ you'll feel rotten
3- bring facial wipes, moisturizer, hand cream & chap stick. Flying long haul will dry you out like beef jerky. Take off your makeup on the plane, moisturize your face, hands, arms, everything. Give your face a quick clean with a wipe now & then, you'll be amazed how much fresher you feel (and how much gunk comes off your face).
4- brush your teeth - if you only do one thing on the flight; do this. You're going to be so jet lagged and out of synch with your Oz timezone, it is only natural to slip out of your teeth cleaning routine. An hour before you're due to land, go in to the loo, brush your teeth & you'll feel like a new woman.
5 - You look very slim in your avatar so this may not be a concern for you, but check out seatguru.com to pick a nice seat, some seats have immovable arms that reduce the seat width. I'm a lot heavier than you so this was more of an issue for me, luckily my husband didn't mind my bottom encroaching on his space

If anyone reading this is concerned about getting the seatbelt on; you can ask the air host for an extender when you board; or you can buy them on amazon to avoid feeling embarrassed.
6 - bring warm clothes for the plane - airlines do provide blankets but be aware that is really can get quite cold.
If you have a stopover for more than 4hours you can call the airline to get a free meal, we had a long stop over in Dubai (8+ hours) and the airline (Emirates) actually gave us meals and a hotel stay. We stopped in Dubai and in Bangkok.
If you're flying into Australia you have to go through an extra security check just before you board the plane, no more than 50 cigarettes are allowed in the country per person and no liquids in volumes greater than 100ml - i.e. - you cannot take advantage of the duty free in the countries you stopover. After you get off the plane in Oz there is a duty free before you go through customs & immigration, the prices are similar to an off license here in the UK but much cheaper than the "bottle shops" in Oz. I don't smoke, but if you do, be prepared to pay $20 for a small pack.
If you are worried about the flight, it's not as bad as you imagine. It is very long, but at worst you will be a little bored, it'll be just fantastic & you'll not want to come home
