Number 1. Lunge lessons! I got tense in the canter too, but took some lunge lessons just to work on the transitions. Not having to worry about control so much really helped me prep for the transitions better until I felt more confident.
Number 2. Break it down and say it out loud! Even now if I feel myself tensing, I talk myself through it in a low calm voice...'ok, at A, we are going to canter, so I am sitting tall, looking forward, I am relaxed, I will open my inside hand and bring my outside leg back and.....WOOHOO!