My instructor would always say to kind of "rev the engine" before popping the clutch. She meant to get the horse thinking forward and stepping energetically, while you briefly hold him (half halt) and then ask for the strike off (timed correctly wiht the outiside hind. always tricky)
When the horse rushes into a trot like that it can be hard NOT to lean forward with him and try to "help" him into the canter. so, avoiding that bumpy out of balance trot in the first place will help. As another person said, pull the horse back to a walk
In a few cases, when the horse was rushing around at a trot trying to avoid cantering, I have raised my hand as if I had a whip and used a "shsh!" to move them on. But I would only do that once or twice to get them paying attention. It's not a good way to set a horse into the canter on a regular basis.