If the only time your cat sees the carrier is right before a trip to the vet, then she will probably run and hide as soon as it appears. Set the carrier up somewhere in your house where your cat might like to rest. Make sure the door is propped open. Keep a towel or small blanket in it, and perhaps one draped over the top to make it an inviting place for an afternoon nap. If your cat gets accustomed to seeing it and sleeping in it, she will be much more relaxed when she has to go on a trip. You can even feed your cat in its carrier or place treats in the carrier for your kitty to find.
What do you do when you haven’t been able to convince your cat that the carrier is a good thing and you need to get her into it for a trip? The most effective way is usually with a towel. Take a large fluffy bath towel and scoop her up with it. If needed, put your cat and the whole towel into the carrier. She’ll probably be happier having the towel to hide under. If you have a hard time holding onto your cat with just a towel, try a large pillow case. You can use it to scoop your cat up, into the pillow case. Most cats will be fairly relaxed when being carried a short way in a pillow case. Set the carrier on its end with the door facing up and lower the pillow case with your kitty inside into the carrier, then close the door. Your cat may come out of the case, or she may choose to stay inside it if she feels safer covered up.
Hopefully you can make your carrier a safe place for your cat insuring that when she does have to travel, it will be with the least amount of stress possible for the both of you.