It's always the little things that trip up a great trip -- lost keys, crossed communications, sauced seatmates. Knowing which disasters are most likely to plague your journey will help you avoid them and make for a seamless trip.
In my experience, the biggest sigh of relief comes when I get off a return flight and my keys are exactly where I expect them to be. In the same way that putting your keys in the same place every day when you get home is the only way to keep track of them, you need a spot in your carry-on bag that will hold your keys for the duration of your trip. It's best if it is a place that you will not access frequently during your trip and is sealed or zippered.
For about 10 years, I drove a car that had a very distinctive appearance; it was large, white and usually the only one of its type in sight. There were some downsides -- folks in town always knew my comings and goings -- but one big upside was that it was impossible to lose in a parking lot. In fact, I was usually able to spot it from the air during my plane's final descent.
The car's successor, a nondescript silver vehicle, is a different story. So these days I write down my parking lot and space information on my parking ticket and put it in my wallet right behind my credit cards. Another possibility: Use your cell phone to take a photo of your parking lot number.
Friends can easily lose or forget all the flight info you rattled off over the phone, and even commercial shuttle companies make mistakes. If you are using a shuttle, taxi or limo service to get to your destination or home, or even if you're having a friend pick you up, make sure you have their email address, and drop them a confirmation note just before your departure with your flight number and scheduled arrival time. This is especially useful on international trips when calling isn't a practical option.
Middle seat: Request a seat change at the gate before boarding. Seat changes at check-in are ever more rare, and waiting until you are on the plane is usually a lot trickier and more political, as attendants are focused on getting the plane off the ground and less inclined to shuffle passengers around. Better to do it officially before you board if at all possible.
Loquacious seatmate: Stow an iPod in your carry-on, and put the earbuds in even if you don't play any music; for best results, pull out a book or newspaper as well, or pretend to sleep.
Crying baby: This one is tricky. Your best bet is to request a seat change; this usually works unless your flight is very full. Otherwise, headphones with music actually playing, or noise-canceling earplugs and a sleeping aid, might be your best bet.
When your plane lands, immediately call the local number of your car rental agency to reconfirm your reservation. You can get the local agency's phone number when you make your reservation (program it into your cell phone so you don't lose it). If there are any problems with your rental, the agency can work on it while you collect your bags and make your way to the rental lot.
Just like your friends who are picking you up, it's easy to forget your flight number, departure time, precise hotel name and address, and the like, especially for a return flight on a trip of any duration. The easiest solution is to print out your full flight itinerary, hotel contact info, car rental confirmations and any other essential documents before departure, and stow them in a safe place in your carry-on for the duration of your trip. Other possibilities: 1) leave yourself a voice mail message with critical information, save it and dial in as necessary, or 2) send yourself an email with the same information, and access or print it out at your hotel.
There are only a few things worse than an empty, snarling belly on a long flight, and only one reliable solution: bring your own emergency rations. (Just keep the TSA's liquid and gel restrictions in mind, and buy your drinks or other non-solid foodstuffs in the airport after you pass through security.)
Pack at least one change of clothing and any essential items -- such as medications and valuables -- in your carry-on bag in case your checked suitcase is lost or delayed. If you'll need clothing or other items while your checked bags are being located, mention this to the claims person before you leave the airport. They may be able to offer replacements or some compensation sooner rather than later. For more information, see Lost Luggage.
Prevention is the best medicine. The Web is packed with live traffic cams, traffic reports, construction updates and the like. Of course, leaving early is the easiest way to avoid traffic; since I rarely do so, I also try to check out alternate routes to the airport before I leave, and bring a good roadmap in case my GPS falters.