Don't lope from the trot all the time. Lope from the standstill, the walk, etc. Do a pivot and lope off. Do a sidepass and lope off. If you're determined to get that transition, I do a lot of backing when my horse anticipates. To lope properly, he needs to be on his butt and not throwing his head. If the horse is anticipating and throws his head, I'll slam on the brakes and back him up until he is round and supple, then lope off from there. I won't let them lope off until their head is down and their body is collected. Sometimes I'll go back and forth for a while until the horse realizes it's easier just to listen. Hope that makes sense.
EDIT: Also do circles at the fast trot until he slows back down.