Love it.
Worst thing you could do? Act / feel like you're not supposed to be there - you
are so you should enjoy it, and not worry at all about 'fitting in'!
As for some tips... hmm...
Look at the timing of the flight and consider your activities on your first day after you land - then decide which leg you want to stay up, and which leg you want to sleep on. Often makes things easier getting more rest on one of the legs rather than splitting it. Also consider when is best to sleep based on the change in time zones, to avoid jetlag.
Dress comfortably, don't worry about going to lengths to look 'smart' on long haul (unless you have to) - if you wear a jacket onto the aircraft, keep your boarding pass in your inside pocket and hang it on the little Skybed metal peg in front of your seat so that a flight attendant can take it away immediately. Take a pen so you can fill out the breakfast form and hang it on the peg before departure as well.
Make use of the ample storage you have available - use the compartments in the Skybed to store reading / pens / phones or other personal items, and leave everything you don't need in the overhead storage bins. Also store your pillow and blankets there until you need them. Put your shoes in the compartments in front of your seat at the bottom, and use the socks they provide in the amenity kit. If available, ask for PJ's and change into them after takeoff (and out about an hour before landing, just after breakfast or before if possible).
Check out the guides on this site and at seatguru for the better seats on your aircraft and try to preallocate one of those if you can.
Pre-charge your iPod/phone/laptop/entertainment devices, but don't forget that you have power at your seat too. Load them with content too, but also bear in mind the wealth of content available on the IFE so check that out as soon as you take off too.
Prepare yourself for landing about an hour-ish before - you have plenty of space, so make sure you've loaded everything except what you need at hand when you until you deplane (and also anything you might need after you deplane - passports, landing cards etc). You can get off the plane pretty quickly, so try to have everything packed, accessible and ready to go once you reach the gate and the seatbelt sign is extinguished.
That's all I got, but I'm sure far more seasoned travellers can chip in.
[Edit]
joeymelb's right about not overindulging at the lounge, but do enjoy it. Also, as I'm sure QF009 will concur, if you're up for it do partake of the pre-flight drinks, and the drinks services throughout the meal service (and of course, the nibblies and drinks at the J bar) but if you do, remember to compensate with lots of water / coffee / bathroom breaks. Keep it limited to the meal service (and maybe one or two others) though, and definitely don't get too drunk.