Always sit the trot for a least a few strides before asking for a transition to canter or walk.
It is common for riders to accidentally teach their horses undesirable behavior by releasing the pressure of an aid at the wrong time. Be mindful of only rewarding behavior that you actually want.
"Incorrect training is as if you buttoned a sweater wrong. In order to fix it you must unbutton it and start over again." ~ Reiner Klimke You cannot just add another layer over the top and expect it to be right!
Horses speak through actions. Try to listen to what they are telling you.