This is what I usually do:
I begin the canter on a 15 meter circle (which is pretty hard for Jesse, he's a brute). Once I've got a decent canter I make it into a 20 meter circle. Then I begin to go down the longside, after a quarter I do a 20 meter half circle. Then I go down half the arena. If that goes well I'll push it to 3 quarters. If that doesn't work out I go back to half and slowly push it further.
I hope this can help.
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie