There are a lot of different ways of doing this. Most of it is the preference of the trainer. Different trainers have different ways of asking for the canter.
In general, I find that the best method is to first tilt the horse's head slightly to the outside. This throws the inside shoulder out a bit, which will help you to get the correct lead. Next, sink your outside hip into the horse's back, and then move your foot a couple of inches back, touch the horse with your heel and kiss. Some horses are also used to hearing the word "canter" so if you aren't getting the message across with the kiss try that. Lazy horses (like mine) may take a little more persuasion, and horses that tend to anticipate and jump into the canter may take a little more subtle cue. This method is the standard method as far as I know, though some trainers prefer different strategies. I once worked with a trainer that tilted the horse's head to the inside of the ring in an effort to balance it, but I personally found that this often caused the horse to pick up the wrong lead.