You dont say how old you are or how long it is since you first rode or last rode?
Carthorse is right that lessons and lunge lessons are the best preparation for canter. All I would add here, particularly if you are on your own and planning canter is to treat yourself as you would a pupil. Make sure you dont hurry to canter before you have the basic building blocks of balance and control in place in trot.
Many of us on the forum took a long time to learn to canter. There is all sort of pressure on adult riders in a RS to canter a.s.p. - in order to ride more cheaply with a group or to hack out. And the pressure to canter causes stress and misery as well as ruining your pleasure in horses. Take your time.
But let us suppose that the real problem for you and about which you are asking is losing your balance on corners in canter? If this is the case that is a common and specific problem. It is really best sorted out in a good lunge lesson, cantering on a circle. Because cantering round a corner is like riding the arc of a circle. (Richard Davidson talks abut this in his DVD on enjoying dressage). He says that the length of the arc approaching the corner should match the length after it before you rejoin the track. Once you are happy cantering on a circle (balanced and not twisting your body, ) you could try transferring that sensation to the corners of the school - riding very gradual corners.
One does not canter a right angle except in very advanced pirouettes!
Shortening the arc at the corners of the school i.e. leaving the track later and rejoining it sooner - will depend on your horse. Some riding school horses are not able to canter small circles (or tight corners).
