Earplugs + Earphones
Do not travel without them. Earplugs do wonders to mute crying babies, airplane engines, and general human nonsense. Use your earphones as back-up, and be sure to download a few boring podcasts to help induce sleep.
Facemask + Scarf + Hat
Create your own personal cocoon with a triple layer of travel sleeping mask, scarf, and hat for complete black-out conditions. It can feel a bit silly and overdone, but remind yourself - it's not about appearances on a plane, but comfort.
Rx, Alcohol, Soothing Scents
Having a cocktail helps some sooth their nerves. Some love the aid of a sleeping pill.
It is impossible to find a comfortable and sustainable position for sleep without one. Find one that is a bit more malleable than the traditional u-shape neck pillow as the added bulk at the back of your neck can be more annoying than useful.
As a tall person, this is my particular burden. Aim for a seat upgrade when you can or take advantage of Exit Row seats when possible. Bonus if you are sitting next to someone you know well - you can occupy a bit of his or her space too.
FOOD + DRINK
Pack Your Own
Do not depend on the beverage cart for your needs. It will disrupt any good sleep you might catch during take-off. At the very least, bring a water bottle and some snacks to tide you over.