I'm just learning to canter, too.. after a year riding. :P It really helps if you're on a slower horse with a slow, rolling canter. It'll take you a while to get the 'sliding' of the canter... it took me about four lessons to get it on Russel-- our slow, 'dead' broke horse. Plus, if you REALLY get your trotting down, that will help.
I started cantering on the lunge. Even though I'm an English rider, I put on a western saddle. My instructor put me on a lunge line, and I just held the horn, kept my heels down, and tried to relax enough to slide. Then I weaned off of the horn, and when I was of the horn, I started cantering 'alone.' I'm now even cantering on the horse I lease, who has a much faster canter. It's fun!