Here are some thoughts from our staff (who have shipped a LOT of pets both internationally as well as domestically).
There are newer regulations regarding snub-nosed breeds such as mastiffs, pit bulls, bulldogs etc. that require them to travel in metal dog crates.
Pets cannot ship in see-through wire kennels. They'd feel more anxiety in those, anyway. The less they see the better they like it.
Purchase your carrier at least two weeks before travel so your pet has time to get used to it.
Give him chew treats and positive experiences at home so he enjoys his time in the crate.
Many airlines require that the crate be tall enough for your dog to stand up in a natural manner ie: the total height of the kennel be equal to or taller than the total height of the dog. However, if it is too large, he can be thrown from side to side and possibly injured more easily than if he is in a crate that is just right.
Sometimes it is the country to which you are flying that requires extra room for dogs inside the crate. For instance, England and Commonwealth countries require enough head room for the dog to stand and not touch the top of the carrier - not even his ears can touch.
Have your rabies certificate prepared in advance.
Most travel requires a valid rabies certificate (within 1 month to 1 years depending on your destination) and a veterinary health certificate within the past 10-30 days (again depending on the destination). The airlines should be able to give you this information. If you can't get it from them, call the consulate of the receiving country.
Some countries and US States have long, arduous, and expensive quarantine. Hawaii, Great Britain, Australia, and other countries have up to 1 year.
Take away all food at least 12 hours prior to travel and water 4 hours before travel. (If you'd like him to have a small amount of liquid to 'wet his whistle', fill his water cup with water and freeze it. It will melt over travel time and give him a small drink without spilling.)
He can also have a natural bone or rawhide chew to keep him busy during the flight. Send a snugly toy with a young or particularly social pup.
If your pets are high strung or nervous, there are a couple of all-natural calmatives or tranquillizers that might help. You want him to be just drowsy, not asleep. Cats don't travel as well as dogs sometimes. However, if you are going to use a calmative or tranquillizer, experiment with it BEFORE you travel to be sure what the effect will be. Cats sometimes react just the opposite of the way you would expect.
If your pet will be travelling in the airline cabin with you, try to exercise outside the airport before check-in and between flights. Don't feed during the trip although a SMALL amount of water may be OK if the pet will be allowed to relieve himself at least every 3-4 hours. Take paper towels or other clean-up materials in case he has an "accident" inside the airport. Take a bottle of his home-bottled water.
If the pet is Travelling in the cargo section, it is best if you arrange for counter-to-counter service so you can check him in and pick him up at the ticket counter with your luggage. If he is Travelling without you, you will probably have to leave him at the Cargo area, but you should ask if you can drop him off at the luggage counter.
Expect that you will need extra time for drop off or check in and be prepared.
You can also expect that he may be sitting on the tarmac for an hour or more before he is loaded into the belly of the plane and after he is unloaded.
The airline personnel will NOT open your dogs crate for any reason. If you want them to water him during the flight (not much we hope), then use the cups that come in the air travel package purchased at the same time you purchase the crate. They can stick the snout of a water can through the door mesh and fill his bowl.
We feel it is better to affix a water bottle to the inside of the crate door with a few ounces of home water for your pet to lap during the flight. That way you can give him a little moisture but can control that he doesn't get so much that it fills his bladder and makes him uncomfortable.
Pets are not allowed to travel when the temperature at either end of their journey is too high or too low. This makes travel safer, but can be inconvenient in Summer or Winter in some parts of the world. Plan way ahead.
The cargo area of the plane (where you pet travels) is temperature and pressure controlled, much like the cabin. He will be fairly comfortable. It's the beginning and end of the trip or during changes of planes that are the most stressful.
Most airlines don't seem to like to handle pets, no matter what they say. You must have your "ducks in a row" and do most of the work for them. If you're prepared, both you and your pets willl be more comfortable, and you'll minimize anxiety, frustration, and airline mistakes.