transitions

["Lysane Cree" <cree_l@LSA.Lan.McGill.CA>]

I was wondering if anyone could suggest some exercises or work I
could do with my horse to help him make his transitions between walk 
and trot and trot and canter better. When i ask him to change gaits, he 
will tense up and want to speed up, and so raise his head and hollow 
his back. Once he is in the trot or canter, he will settle into the 
gait just fine after a stride or two, and lower his head and round 
his back more. He used to throw his head around a lot and be really 
nervous when I first got him because people had been hard on his 
mouth, but he got over that. And he is not really fighting my hands 
when he is raising his head to make a transition, but it seems as if 
he is not 'balanced' enough or bringing his hindlegs underneath him 
enough (??). Any ideas? He does not have a problem with slowing 
down and changing from a canter to a trot or trot to a walk.
Thanks for the help,
Lysane and Buck