This one comes from the I’ve-been-taking-too-many-trans-oceanic-flights department.
Taking a long flight? I find that one of the things that most fatigues me is feeling gross after sitting for hours on end. Though I would recommend you bring a change of clothes with your carry-on in any case (what do you do if your luggage gets lost on the way?), I make it a regular habit now when traveling for more than a few hours: bus, train, plane, car, etc…
On my recent trip from Orlando to Dublin I had a direct flight and then a bus ride to Cork, Ireland. Since I had been up all day in the same clothes before my 8:20 pm flight, I changed into comfy shorts and a tee just before boarding and felt all refreshed before the long haul. It made a whopper of a difference in how I felt sitting in the cramped seat. Additionally, take your toothbrush and toothpaste with you so you can brush on the plane if the meal leaves a less-than-satisfying taste (or stench 😵) in your mouth.
If only I had done this on all those hot summer trips to Kansas growing up…