In days gone the canter wasn't worked on until the horse was able to perform the lateral movements in trot. In La Guérinière’s time the canter wasn't discussed until after passage and piaffe!
A prerequisite is that the horse is tracking true; his left hind following in the same track as his left front, right hind following right front. (see Straightness)
Assuming that your horse is ready for canter, there are several things that you could be doing to upset the horses balance, such as:
Tipping forwards in the shoulders, thus collapsing your front line
Lifting your seat
Putting too much weight to the outside
Not *preparing* -with your whole body - for the transition
Getting ahead /left behind the movement
Allowing the trot to deteriorate
You may also be *abandoning* him as he goes to canter. This usually happens because the rider has given the reins away in a mistaken attempt to "give him some room". Have the sensation of only 'giving' through your joints; so that your shoulders, arms, hands, wrists and fingers remain 'soft' and 'receiving' of all that power you've just generated.
You have to be able feel the footfall sequence, being able to accompany the horses movements with your seat /hips so that you can influence them. When walking or trotting, the horse advances his hind legs equally. In canter, he changes to an asymmetrical stride pattern, whereby the inside legs advance more than the outside ones. To make this change, we need the outside hind to remain on the ground for a fraction longer, whilst the inside calf asks the inside hind to lift off sooner and to reach further forward than it does in walk or trot.
Remember also, that *the* most important aid is our seat. That must remain glued into the saddle whilst giving aids with any other part of our body. If we come off the seat, even very slightly, this in itself can destroy the clarity of any other aid we may be giving at the same time. So do your best to remain 'softly' glued (without pushing downward) through your seat as you give your canter depart aid with your seat/hip bone and inside calf.
Think of the aid beginning with the outside leg and shoulder. The outside leg goes back slightly, from the hip joint, not just the knee and prepares the horse for the strike-off aid with the inside calf. Take care that the outside shoulder doesn't fall back. This is done through a subtle toning in the abs and back and not by advancing the shoulder itself. The latter will lead to tension through the arm into the horses mouth, to which he will react by tensing too. This arranging of the shoulders through the waist is for all turns not just the canter depart as it will also help to keep the inside arm soft and allow for subtle aiding with the fingers and wrist with regard to flexion and bend.
A good way of checking whether you'll get a nice depart:
As you are trotting round in a nice active, forward rhythm, think not of preparing to canter, but of preparing for a walk transition (this can help with the mental block some riders seem to get when thinking about canter). When the horse feels as though he could easily move forward into walk, you, instead, give the canter depart aid (see above). This helps the horse to remain balanced within itself while going from one gait into another.
You need to keep the horse balanced under your seat, in terms of his energy and rhythm. This way the transition begins under your seat and not out in front or behind it. It also feels much more uphill as the horse takes the weight back rather than rushing unbalanced onto his forehand. Check the horses balance (readiness) with a half halt. This may help with the psychological block some folks get when thinking about canter.
Did the half halt get the required response? No? Ask again, and again - if necessary- then, when the horse 'feels' as if he could produce an effortless halt or walk from the trot, is the moment your horse is balanced and only then can you give the correct canter depart aid.