OK, my cat has crossed the atlantic a few times, poor kitty. But here's what to do...
Get all the cat's shots and health certificates in order. Check with your vet, they'll know exactly what you need. I would advise against drugging the cat. That can have adverse effects at high altitude.
Contact the airline you're flying with and get all the info pertaining to flying a pet. Also, do this VERY SOON. When you call, tell them you're transporting your cat, but you'll like to take it with you in the cabin. You have two choices, the cat can be shipped in the cargo hold (it's heated), or can travel with you in the cabin. However, most airlines will only allow one pet per cabin, for obvious reasons, that's why you have to reserve this NOW.
They will have you pay a fee, usually between $50-$75. Don't get upset. This is actually an insurance fee. If anything should happen, you and your pet will be covered. If you sneak the cat onboard, and something should happen, you'll face ALL liabilities.
Buy the bag specified above, it's much better than a kennel. It looks like a duffel bag with a mesh for providing air. Cats prefer it because it's more comfortable, and it won't draw a crowd at the airport.
When you board, simply put the bag under the seat in front of you. My cat was very well behaved for a 12 hour flight. You'll be surprised that they don't need to eat for the trip. Just make sure you don't feed them a few hours before the trip, and make sure they use the cat litter before leaving. You really have to time this very well. It's no fun taking the cat on an airplane if it's bladder is full. Rest assured however, that very few times will they really really need to go. They can hold it in for a surprisingly long time, and they can go a few days without eating. More than likely, they'll just sleep the entire trip. My cat woke up a few times, and I just put him in my lap (in the bag) to comfort him, but I had no problems whatsoever, and I had to transfer and take another flight.
It's not a bad idea to take a temporary cat litter with you. You can buy these at Walmart or Target. They're basically litter boxes made out of plastic with the litter sealed in. They're made to be portable, and you can easily fit one in your carry on bag. If push comes to shove, you can easily open up the portable cat litter box in the airport bathroom.
Most European countries aren't very iffy about pets, except of course, for countries like the U.K. Please find out the country's policies before going there. THe airline will usually have this handy when you book your flight with them. Most times, the customs agents won't even realize you have a cat if you have one of these duffle bags. Most times, they just won't care. Nowadays, in wake of the recent attacks however, be prepared to undergo extra scrutiny. In the U.S., they have ALWAYS had me take the cat out of the bag, put the bag through the scanner, and have me and the cat walk through the security screen together.
Any questions, send me an email.