Sleep kits can be purchased in most airport gift shops these days, or you can build one at home to keep with your luggage. The basics for a sleep kit include a U-shaped travel pillow and an eye mask to block out sunbeams bouncing off the clouds below.
You'll also want to block out as much sound as you can, and earplugs can definitely help with this. If you've got the extra cash, noise-cancelling headphones can make a world of difference, too. They can negate outside noise (crying babies and aircraft engine sounds included) whether you're playing music through them or not.
A small blanket or large shawl is the final item you might want to stash with your sleep kit. It's nearly impossible to grab some sleep when you're shivering. Airlines used to give out blankets regularly, but these days, it's often another item they can charge you extra for. So if you've got a red-eye flight or perhaps you're crossing a few time zones, it's best to bring your own sleep kit -- but keep it light.
Speaking of keeping it light, take a look at the next tip.