Practise, practise, practise. Make sure your horse is square before mounting and dismounting, and halt before each break. The more you halt squarely, the more likely it is to become a habit.
Sit up tall and check you are sitting straight and your weight is even on both sides.
Every time you adjust the halt, walk forwards not backwards, so your horse is thinking of going forwards to the next movement.
Keep your legs wrapped around your horse during the halt and teach him to move forwards when you release. This helps to get a positive, powerful move-off.