Personal air purifiers, gizmos that you wear around your neck, are a great way to keep some of the cruddy cabin air at bay while giving you a constant dose of moist, ionized air. This treated air can make you feel a lot more energized and awake at the end of your trip. You'll look like a dork, but it's worth the sartorial shortcomings. Be advised that these devices can irritate some people's sinuses, so your mileage may vary. See DLT's article on Personal Environment Zones for more information on PEZs.
Most women already know the benefits of moisturizers, but few guys ever touch the stuff.
On a plane, you should. You should also carry a good eye cream for grueling flights that leave bags and dark circles. Carry some eye drops, too. Even if your eyes aren't particularly bloodshot, they will be dry. But it'll all be worth it the first time you feel like a human being rather than a side of beef as you head to that big meeting.