Well, with Artie, it was his skittishness...he is still jumpy.
I really have not found anything to help him. I so keep a box, and a paper bag on the floor for him to hide and feel safe.
I also have made him a 'tent' which he loves to sleep in, especially when I am not home--in addition to ontop of the bedspread in my room.
He is one spoiled kitty!
When I have to put him in the carrier it is fight and chase time! I have to close off the rooms, catch him and quickly get him in the carrier!
I have found that he seems a bit calmer and quieter if I keep the sides covered and if I do not talk to him, that is when he is in the car..
I did buy a calming collar, but it had so much 'stuff' covering it that it flaked off all over the place and Artie was afraid of it....more wasted money!