I get Amber working well in her trot. I like to have her on the bit, but if she's got decent bend through the body and is using her hind end, I find that acceptable right now - she has not been in dressage training long, and was a trail horse before, so bending and contact are all new for her.
When I have that trot, I commit myself to the transition. You have to mentally commit yourself to ask, otherwise you half-ask and get a half-response.
I half halt first, then give a little squeeze on my inside rein as I push her with my inside leg into my outside rein. If I am posting, I sit before I ask and push her with my seat bones, putting weight towards the outside seat bone.