Lunging over poles
Get them to engage and collect up
When riding get them to work long and low but not having swinging reins, keep a contact and once again poles poles poles
If possible go for long uphill hacks or trotting up steep hills
I have worked with ottbs that won't build topline, One thing that really helped me was to do lots of lunging in side reins over poles ( helps the horse step under its self building topline) .Also under saddle with a German martingale. You have to make sure that you are not cranking the horses head in so don't make the German martingale to tight. Draw reins also work but are not a easy and comfortable for the horse.