You can decide the number of strides you want to do in each allure and then keep it for example : 5 strides of walk, then 5 of trot, then a 5 seconds stop, 5 strides of walk, 5 of canter... This kind of exercise will make you and your horse more reactive.
If you use a dressage arena you can use the letters to practice precise transitions, and if you don't, use repairs like a tree or a pillar.
Another way to improve transitions is to improve the quality of each allure.
Once you and your horse are reactive and do transitions in precise points you can try to do them in shoulder-in or sidewalk.
Hope this helps and sorry for my approximative English :)
Do a transition at letters in a dressage arena. I do walk - halt at every letter when my horse isn't listening to me but you could do it at whatever letter you want
Doing transitions themselves the best way!! Just practice them over and over.
I normally use a dressage arena to practice flatwork and transitions. This way I can pick a movement and at which letters the transitions are going to be done. Ask for a transition upward when your horse gets lazy, on the other way ask for a transition downward if he/she gets too excited. The goal is to ask for a transition once and have it done at the chosen mark.