Definitely do a search of this Training forum, and maybe the Riding and Dressage forums as well. Tons of excellent info.
In the meantime, lots of transitions, as kitten_Val said. Don't make 1 lap around the arena without changing something about your horse's gait, direction, speed, or balance. Something else that you might find helpful is backing up 3-5 steps after each halt. It gets them stopping on their haunches more, and "thinking back."
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown