I know a couple horses like that. One particular mustang I ride can trot for days without breaking into a canter.
For him, I do the ask, tell, demand scenario.
I ask for a trot, and at the corner of the arena, I ask for a canter with a kiss and a slight kick while sitting and not posting. If he doesn't listen, I wait till the next corner, kiss and kick a little harder. If that doesn't work, I either slip on my spurs, grab a crop or give him a good whap on the rear to show that I mean it.
He is a very stubborn horse, so sometimes he just needs a bit of incentive to get going. : p