I've flown to Perth, yes it's one long haul!, although having missed our connecting flight in Singapore, it was broken somewhat, so that final leg was maybe only 5 hours!
Layering clothes wise, try and go for seamless trousers like the yoga ones you mention, walk about whenever possible, ankle flexing exercises, not too much in the way of alcohol, keep hydrated, compression socks on long haul, they are uncomfortable, but possibly wise. Something to read, hopefully something to watch. Premium economy is worth it. Many years ago now, but Singapore Airlines were great. It's really nice to freshen the mouth so one of those mini tooth cleaning kits if you can get one. Skin dries out, so maybe include facial moisturiser or a water based spray.
Good luck and bon voyage!