One thing that may help a lot if you know that you have an impending veterinary appointment is to leave the carrier around so that your cat may get 'acclimatised' to it. Leaving some bedding in there that your cat has used previously and placing in a spot with the door/lid open so that the cat may wander in and out can be a positive move.
Interestingly I came a across a small study that showed that the act of allowing a cat to get used to it's carrier prior to a trip to the vets really helped to reduce stress for our feline friends.
You can read about the study here
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