Ask for flexion and counter-flexion as you ride around the arena on various lines and shapes.
Avoid asking for too much bend in his neck and shoulders, as this will put the horse off-balance and increase the chance of falling on his shoulder in an attempt to secure his hindleg. Instead, slightly open your hand and wrist on the inside of the flexion, and rely mostly on your legs to direct the bend. Reward your horse when he responds.
Do not change bend too often. Make sure that the bend is well established and correct, and only then change the flexion.
NOTE: Do not pull on your horse’s mouth to achieve flexion. You want soft and gentle flexion (focusing on the poll), not a tense and forced movement.