I would not hold the back of the saddle - dangerous. I know you are going to hate this, but the answer is in your trot! Learn to get deep in the saddle during trot and it will be totally natural in canter. Do rising trot, sitting trot, rising, sitting - and lots of it. When you go to canter, sit trot for about 10 seconds prior to asking for canter, and I know this sound rude but.... push your boobs out and lower your shoulders. Then just sit and try to 'go with the movement' - be as relaxed as possible. Hope this helps!