People who look so relaxed at canter , like they are doing nothing, are actually doing something; they are engaging the core muscles, and moving WITH the horse, with their abdomen. They may have relatively loose legs, but their core is working.
Some people use the anology of 'painting' the saddle with your butt' , which would require you to move your but along, rythmically, as the horse moved.
That can help.
The thing that helped me, and others, the most is to think of going DOWN with the horse, when the horse is going 'down' in the canter stride. I've described this a dozen times on this site, but here goes . . . . The horse's canter has basically 3 beats, with a moment of rest before repeating those 3 beats again. Let's call beat#1 as the strike off beat; the outside hind hits the ground. #2 is the inside hind and outside front hitting at the same time, and #3 is the inside front, the 'leading' leg, hitting the ground with the other legs coming off the ground as . . . the horse has a moment of suspension and repeats the sequence.
At #3, when the leading leg is hitting the ground, the horse is in it's most 'downward' position, as if it were going down a hill. You need to canter a bit, find that position by feel, and start really thinking about your pelvis following DOWN with just that beat of the canter . All other beats allow your horse to move you, but on the 'down' beat really think about your seat bones and pubic bone trying to go all the way down into and through the hrose's body, to the ground.