I asked my mare to canter up hills to get her to canter on flat ground when I asked her to. It took a few times but she was cantering by the end of the day
I've heard of people using a very low crossbar and pushing them into the canter as they go over. I've only had to do it a few times and it's worked for me.
I have in the past used a pole in the corner of the arena. So that you can trot up to the pole ask when they go over the pole and into the canter. This is just a stepping stone as you can start to canter without it but, I have used this and it has helped