Fitness and correct sequence of aids are the two keys for getting a prompt, correct canter departure. Young horses also tend to run into their canter because they don't yet have their balance under a rider. Being heavy on the forehand tends to produce the running into canter.
The fix for the fitness issue is lots more wet saddle pads, with a lot of transitions and some hill work.
The fix for young/unbalanced/heavy in front is the same.
The fix for correct sequence of aids is well, to make sure you're asking correctly and to have a competent person on the ground to correct you. A lot of people ask for the canter by leaning forward and squeezing with both legs; essentially, asking the horse to run into the canter.
Practice walk-halt-walk and trot-halt-trot transitions until the horse is really prompt about moving forward of your leg. Then practice moving from a working trot to a slow trot and back. Then ask for the canter from the slow trot - you're much more likely to get a prompt transition.
And a personal pet peeve - ask for the canter either from a full seat or jump postion. Posting to the trot while asking for the canter confuses the horse and reinforces the trot rhythm and pretty much guarantees the horse will run into the canter.