I recently flew into Phoenix and had to endure two time changes, a connecting flight and more than 10 hours of travel each way. If it weren't for my strategically packed carry-on bag, I woulda been crazy-bored and ferclempt.
I've been guilty of overpacking, and nothing is worse than lugging around a huge carry-on from airplane to airplane and gate to gate. Feel free to overpack that suitcase (that you're checking), but pack as lightly as possible when it comes to carry-on luggage. Not only do you have to schlep it everywhere but it will need to fit into those sometimes-tiny overhead bins on the plane (and not all overhead bins are created equally – trust me – I've had to check my bag at the gate because of this and hold up entire flights).
You'll want to have a few sources of entertainment stashed in there – these can vary from a compact laptop to a book, magazine, newspaper, iPod (source of music), puzzle book, deck of cards, or even a handheld video game.
You'll also want some "defence" items, things that will ward off pests such as germs, hunger, and that chatty seat-mate. Earphones/earplugs are the perfect way to "tactfully" let your neighbours know you're not interested in hearing their life stories en route to your destination; they may also help you catch some Z's and drown out talkative girlfriends or crying babies. Hand sanitizer, a bottle of water and a few Vitamin Cs never hurt either – in fact, they're a must when I travel (since I always seem to catch a cold after flying somewhere – damn circulated plane cabin air!). And we all know that flights are zero-frills these days, so other than a handful of stale peanuts or a weird cookie, you won't be offered food – have a few healthy snacks stashed in your bag, such as pieces of fruit, granola bars, nuts or seeds, a muffin, or even a sandwich. And don't forget gum or candies (they can also help ward off ear-popping when you takeoff and land).
Make a quick list before you depart and ensure you have all the main necessities at hand in your bag. A properly packed carry-on bag really can mean the difference between a comfortable and a nightmarish voyage.