I specifically bring my own in-flight attire with me on long haul flights. This includes a sweatshirt, joggers, shorts and flight-socks.
As soon as I get on the plane, I establish my zone of comfort around my seating area, including having all items to had as required during the flight, such as headphones, book, snacks and refreshment. Next I de-shoe and take whichever in-fight attire I deem most appropriate for the journey into the toilet. As a rule, this is generally shorts and flight socks. I change out of my clothing into t-shirt/sweatshirt and shorts, and change my socks. I settle back into my seat and make my entertainment choices for the flight (no, this does not involve custard).
There was one occasion flying back into London when I had forgotten my shorts so improvised by wearing my blanket and taking off my jeans. I awoke to the stewardess attempting to remove my blanket as we were about to begin the approach to descent. A quite vicious tug-of-war ensued which was tempered by my then girlfriend explaining the situation to the stewardess, which I hadn't been able to in my newly-awoke mindset.