I never sit before canter anymore, all my horses ahve learned the way I do it.
My aid for canter is when I get to the sitting part of rising trot, I half halt, sit really deep and drive with my seat, while applying my outside leg.
You have to make sure you have a good quality trot before hand, as well.
After a while, my horses got so fine tuned that I don't need leg, I just sit super deep for a stride in the trot and they pop int a lovely balanced canter, no worries.
Another thing that helped my canter transitions was walk to canter, it really makes them more responsive to your leg, and teaches you the correct timing in regards to their footfalls.