We LOVE to canter! My Rocky walks, gaits, paces, trots, canters and anything else you can think of, really. She has several cantering speeds and gaiting speeds. She knows when I'm asking for gait because I collect the reins and ask for speed. When asking for the canter I loosen the reins a bit and ask for more speed.
Also, if you move your body in the saddle like you are cantering (or whatever gait you want), the horse will often fall into the gait you want quite easily. My girl, now 5, is probably still considered green as she hasn't been ridden a whole lot, however she is stable and well versed in what she knows and can be ridden by darn near anyone.
Other than the above, I don't have any set cues for the different gaits/speeds. I just ask for more speed or less and give or take on the reins as needed to accomplish what I want. Don't expect to get speed out of your Rocky if you ask for speed with your legs but don't ask with your hands (reins). Hope this helps.