Grip with the lower part of your calf and sit deeper into your saddle so you don’t lean forward and lose your balance quicker
Pretend your feet are being pulled by strong to the floor and try to move your seat as best you can to your horses canter even if it requires you rocking your hips a little
I sometimes try to close my eyes down the straight sides of the ring so i can really focus on my horses movement and it helps me to balance myself and make sure i stay with him.
keep you shoulders open and stretch you belly, grip with your lower legs and thighs and just go with the flow of the canter.
Try to use your knees and thighs to hold on to the horse. Also try moving your back and hip according to the movement of the horse.