Exercises for keeping a calm canter and control of the pace?

My best tip is to practise letting the horse hold the pace and the gait without you having to be there helping or correcting all the time. A good way is to set out cones in a circle. Ride the circle in walk first, influence the horse when you need to, but release clearly and at once as soon as she follows the circle and holds the pace. Once that works in walk, do the same in trot and finally in canter. It may well feel as though it is going badly for a while before the horse realises she should stay on the line and at the pace you have asked for once she is free of any influence, for example on a loose rein.