depending on your horse, some are more sensitive to cues than others. Some are taught the cue to canter differently where as others are dead sided from use of spurs or too much leg (my horse got like that for a while).
I am not certain which way is the "politically correct" way to ask for a canter, but here are different ways that I've seen horse's go into the canter:
1) Bringing your outside leg (the one by the fence/arena wall) back further
2) Leaning back and giving a tiny squeeze with your leg
3) Leaning forward and giving lots of rein
4) A mixture of any/all of the above
My horse knows #1 and #2, but he responds better to #2 because that was the way he was origianlly trained, and since I ride bareback, and neckrein him I find it easier for him to respond to #2 more-so than #1