Sue Morris has a pretty good article on the canter depart that you might find interesting...so instead of paraphrasing her, I'll just give you the link: http://www.classicaldressage.co.uk/h...er_depart.html
What it boils down to is that you need good rhythm and balance at the trot before you can expect to achieve a good canter transition, and that timing is everything. Learn to feel where the horses feet are and when you should ask.
Canter transitions are a lot harder for young and untrained horses than a lot of people seem to think. You can run them into it and if they are somewhat athletic they can pick it up...but there is a science to when and how you should ask to be successful.
I've said this before, but asking for the canter off a circle is a good way to set up a young horse. Anyway, I'll shut up, go read the article:P