Two of my angel kitties – Missy and Trikki – were absolute angels at home, but the poor vets and techs who met them wouldn't know it because while there they were total hellions. By planning ahead, and visualizing a positive outcome both for the drive and the actual visit itself, we got through it without needing an entire suit made of Kevlar. Okay we cheated and used falconer's gloves sometimes!

 

All my cats love to sleep in carriers, so I have one in practically every room. They don't view the carrier as something that comes out only on vet visit days, so there's no negative association there.

The night before

To make the experience as pleasant as possible, we bring our cat's carrier out the day before the visit and spray the inside with Feliway - a pheromone product available at pet supply stores – and close the door so the cats can't get in. Feliway spray contains a high % of alcohol so it's best to let it dissipate overnight rather than spray it on the day of the visit.

Before leaving home

On the day of the visit, we spray a wash cloth/handkerchief with lavender hydrosol or catnip spray or both and place it inside the carrier. At that point, we open up the door to the carrier which immediately piques our cats' curiosity. After all, if anything is off-limits, it's suddenly even more attractive than it was before, so they just have to get in there to check it out.
[Our 17-yr old sweet Missy so terribly prone to motion-sickness:(]

Our 13-yr old sweet Missy so terribly prone to motion-sickness:(
My biggest problem sometimes is getting another cat out of the carrier, so I'll spray all the carriers the night before; that way I'm bound to have one carrier with the cat who has the appointment ready to go.

If by some miracle, your cat is not in the carrier at this point, place him/her in there with soothing words, and the combination of pheromones and hydrosols will keep him/her relaxed during the car ride. Unless there's a chance of throwing up, be nice and give your kitty cat a treat, what the heck s/he going to earn it.

Flower essences

You can make things even easier on your cat by dabbing a flower essence on the paw pads or just inside the tops of the ears where the fur is not thick. You can also add a drop or two to your cat's drinking water. Some essences that are effective for this are Aspen for fear of the unknown, Rock Rose for trauma, or a combination calming essence e.g. Five Flower essence or Bach's Rescue Remedy or Australian Bush Flower Essences' Emergency Essence.
[Naz's Abu riding shotgun with his daddy]

Naz's Abu riding shotgun with his daddy

These can also be combined into a mister bottle which you can spray inside your car before you get in as well as when your cat is in the car with you. If you need to, you can continue to dose with essences when you get to the veterinary clinic.

At the veterinary clinic

Your cat will be able to smell fear and other animals in the reception area but the pleasant calming smells inside the carrier will help reduce his/her anxiety level. If at all possible, sit away from other species of animals, and keep your cat's carrier facing away from them but towards you. This will allow you to speak to your kitty soothingly, and keep him/her calm while waiting for the veterinarian. Sometimes it helps to cover the carrier with a cloth so your cat can't see another animal through the holes/vents, and feels more secure with just you in her/his field of vision.

Homeopathic remedies are extremely effective, and increase your chances of a smooth, happy (okay, maybe that's pushing it with some cats, but you know what I mean ) trip. Needless to say, remedies should be based on the individual cat and situation, but since cats typically have a certain set of reactions at the vet, I suggest you take a look at the descriptions for Aconite, Arnica, and Chamomilla:
http://homeoint.org/books/allkeyn/index.htm