Take care of yourself
First of all, make sure that you stay hydrated. Take an (empty) refillable water bottle with you and fill it up at a fountain before you board, as flight attendants will only come by once in a while to offer you a drink. If you're flying for more than 4 hours, you will likely be served a hot meal, but bear in mind that options for food restrictions are often limited to veg or non-veg. Take a few non-perishable snacks with you such as dried fruits, pretzels, or crackers to munch on while you fly.
Another thing that shouldn't be underestimated is the close proximity to hundreds of other people in the cabin, a few who will likely be sick. Post-flight colds are quite common, so stock up on hand sanitizer, tissue, and wet wipes. A cloth or paper facial mask can also help germs at bay.
Keep your toiletries and medicines in your carry-on bag (Include your tooth brush and mini tooth paste, eye drops, sleeping pills, allergy medications, prescriptions, etc.) It may sound silly, but not being able to brush your teeth for 12 hours is a nightmare, and if you need to take medicine regularly it's important to keep it on your person, in case your luggage gets lost.
Lastly, take care of your skin. Dry cabin air can cause breakouts, especially if you already have sensitive skin, so take some facial wipes and a small container of your favourite hydrating mask and moisturizer and apply it before and after takeoff.