The best thing to do is wear layers so that you can adjust if you get hot or cold. Also try to wear things that don’t get crease easily so you wont be fixing yourself to not crease or feel like you just went through the wash/dry cycle by the time you get off the plane. Wear shoes that slip on and of easily so you don’t feel constricted in lace ups for so many hours. The blankets they give are really not warm, the pillow is tiny. So if you can stuff your favorite pillow into your carry-on you will sleep better.
If you are in business class they will give you ear-plugs and an eye mask. If not buy your own to help you sleep. You might shut your light but your neighbor might not. You might want to sleep and your neighbor might be very chatty or there can be a baby crying next to you the whole way.
If you don’t mind changing, you can always take a change of clothes in your carry-on and change in the bathroom half hour or so before landing. Don’t bother worrying what other people think or do. Everyone does what they are comfortable doing. Men will fall asleep in their suit on the couch at home anyway, so don’t go by that.