Well, one, my main advice in cantering is simply, dont hold your breath. When you do, you become tense. Cantering is fun, smoother than trotting and good for the horse, so enjoy it. A way to become less tense is to smile. Simple as that. Smiling relaxes the body even if you don't realize or feel it. Also, kind of make a rhythm with it. "One, two, three, one, two, three, one, two, three..." because you can't hold your breath and talk. Think "down and forward" with your seat. If you can get someone to put you on a lungeline, practice the canter with your eyes closed, then with your arms out, then arms up, and finally no stirrups. You may not want to do this all in one ride, but remember, you have to advance forward or else it will get harder each time. Cantering is fun, so take a deep breath, relax, smile, and enjoy it. Good luck!