[MyBoyPuck]

Part of it is timing. You want the horse to be able to step out of the canter and into the trot. The first beat of the canter stride is the hind leg striking off. The beat before that is the moment of suspension. If you still your seat and half halt during that moment, your horse should downshift into trot with that hind leg instead of continuing to canter. Make sure you're sitting square and tall with your core engaged so you don't fall forward. I'm sure the more experienced dressage gals will have more to add. That's all I've got.

 

[~*~anebel~*~]

There is a reason this transition is still featured in the GP - it is very difficult to do correctly.
For a training or first level horse to be able to do this transition without "falling" or doing anything else weird would get an 8 or 9.
Now - to achieve this we first of all need to change the thinking of the rider. One is not doing a transition out of canter - but into the trot. You are not ending the canter, or reducing the energy in any way. Imagine that you are in a walk and asking for your first stride of trot. You want the horse to spring forward energetically into the new gait - we want the same thing from the canter.
This means that the reins are acting passively - they are not pulling to "whoa", only gently feeling and "recycling" the energy created from behind - as they are usually doing. The legs play an active role in maintaining the energy coming from behind - as they are usually doing. It is the seat which is regulating the stride in the canter which begins to ride a trot rhythm. In canter we have a nice 1---2---3---1---2---3 beat which the seat is regulating, when we want the transition we stall slightly with the seat and begin a 1-2-1-2-1-2 rhythm. So infact, the rhythm of the seat is speeding up.
This takes some time to get used to. However, in the upper levels the legs and hands are doing so much that they don't have the time or space to be regulating gaits - so we must begin to teach the horse to be regulated by the seat early on. Eventually - all transitions will come almost entirely from the seat changing rhythm!!

So - good luck and if you need to use a rein or leg aid to enforce the aid at first then do so in an educational manner - always reward correctness and remember - perfect practice makes perfect!! You must perform well and consistently before your horse can.