It's always a good idea to carry all your essentials in your carry-on. This includes things like electronics, medications, toiletries, etc. That way if anything happens to your checked bags, you will still have everything you need with you!
Having a fully charged power bank is essential for long-haul flights so that you don't run out of juice on your laptop, phone, or other electronics. Just make sure that it meets the airline's regulations and fits into your carry-on!
Whenever I travel (particularly in the summer), I ALWAYS wear clothing that is comfortable and warm. I usually wear comfortable running shoes, my Adidas joggers, a short sleeve shirt, an extra thin long sleeve shirt, and a jacket.
I always bring a night mask and ear plugs on a long flight because when I want to get some rest, it helps to block out the light and any noise that there is.
If you are able to, try and get a good nights sleep the night before that way just in case you can’t get any sleep on the plane, you won’t be so tired when you reach your destination.