Repetition. Repeat and eventually it won't be so exciting :p
the lunge is for bucking & playing around.
If it really bothers you, you can just double the time you do trotting to get rid of that freshness and then when you see your horse is calm try and ask for canter.
Sometimes you have to just point it out to the horse that you dont like it and what i mean is use your whip or sharply pull on the lunge. Ask your horse to go back to trot straight away and ask for canter again and do it as many times as you need to untill your horse goes smoothly and calmly into canter and until it understands that this is the right way to do it. Your horse must respect you.
If you feel your horse is not fully respecting you, have a look at some exercises based on horsemanship ;) :)
It's probably just extra energy, nothing to worry about, mine always bucks in the first transitions after his day off