As being an Olympic Eventer, and training several OTTB's and Extreme Mustang Makeover horses, I would usually start off with a soft and gentle bit. If a horse where sensitive to certain touches then I would keep off of my leg and just use my seat to be more forward in the trot. I would also give the horse a lot of rein so he does not feel that he needs to pick his head up. If the horse was not sensitive to the touch and was able to be ridden with double bit and rein the I would use that and have full contact with the bit and seat. I would also use spurs and do gentle taps and I would not use a crop or a whip when I am riding in a situation like this because that is just a way of telling the horse that he needs to be forward and that may cause him to pick his head up.
I have a young mare currently in training I find with her anyway riding on a soft rein and getting her going off my leg she finds hee own natrual lower outline this was so it's not forced and looks beautiful I might add! ❤