Shoulder in must be done CORRECTLY to obtain its benefits. Many riders think they're doing it right, but have little bend, turning it into a leg yield instead. Leg yielding is useful for suppleness, but lacks the engaging & collecting effect of a true shoulder in. Learn how to do it correctly here!
"Riding is, therefore, an ongoing, never-ending, challenging process. That aspect makes riding so intelligent and significant an effort. One merely strives, never arrives." ~ Charles de Kunffy
"If the rider encounters a problem, he must first look for the cause in himself. Most of the time, the rider is the cause of the problem. Only someone who looks at his own riding critically and is willing to improve constantly and learn more will be a good
Your balance and the strength of your core are what allow your position to be unaffected by your horse's changes of speed.
"The quality of a good canter: round, energetic, straight, cadenced, light. You have to feel in the strike off that the horse lifts his forehand without throwing himself forward." ~ Nuno Oliveira