I usually come on to a l/h flight (whether day or night) wearing something like a shirt/trousers and jacket or jumper. I either have BA-issued or home-issued clothes for the flight, loose top, comfortable but loose bottoms. Not football training gear or Juicy Couture, but I think you get the idea. Lufthansa gives out a very nice F van Laack set. With an hour to go, I change back in my clothes which are neither creased nor have the sucked up airplane+sweat smell.