First, try to make the crate that your cat will be travelling in as pleasant as possible. Place a comfortable cat bed inside or at least a soft towel that smells like home. Crates where the top can be unscrewed and removed work best. This allows the cat to remain in the familiar bottom half while the veterinarian performs an examination, gives injections, etc. Spraying the inside of the crate with Feliway can also help cats relax. Leave your crate out from time to time and place treats or catnip inside so that your cat occasionally has positive experiences with it.
If the veterinary hospital itself is the primary source of stress, consider using a feline-only practice if one is available in your area. These clinics are quieter (no barking dogs or rambunctious puppies allowed!) and designed specifically for cats. The veterinarians and staff also have experience handling the unique needs of their feline patients. Another option is to make an appointment with a housecall veterinarian and skip the trip to the clinic entirely.
Whichever option you choose, remember that even if your cat seems to disagree, a veterinary visit is in his or her best interests.