^ you HAVE to ease them into it or you'll pull them over! Big circles in the direction the horse is set up for (you have to be able to feel your canter leads though, if you look down to check you will be off) and slowly easing into small circles as they slow up. Circles are harder than straight lines for the horse.
Otherwise pushing them on works really well. I had a pony that would bolt if I let him get too far behind the other horses I rode with and every time he did it I would make him keep going past them another 50 metres and then trot him back towards them, he would bolt again, I would push him past them again... it fixed him up real quick!