I try to cue from sitting trot because it allows me more control over the rhythm heading into canter, but I can't sit my mare's working trot to save my life (17hh and huge mover) so I'm as likely to post into it on her as I am to sit.
Inside leg on the girth, outside leg behind, make sure my shoulders AND HERS are turned towards the lead I want her to pick up (left lead = turned left, right lead = turned right), "Ready?" & half-halt, then "CanTAH!" and ask with inside leg. I am not a dressage person, so I couldn't care less if I'll "lose marks" for voice cues. My mare transitions better with them, and a better transition means a more balanced canter.
Fun fact: flying changes are often cued by giving the cue for canter on the other lead :) my mare does them that way, and so have the other two I've ridden that knew them.
MAKORA THOROUGHBRED SPORTHORSES