I agree with Tina Papas but it does take time. I've started to try be strict with myself to make sure I let my horse stretch down in the trot after I ride so that he will eventually start doing it quicker and start off like that. Lots of transitions and make sure you have impulsion.
Maybe riding in low head position and some shoulders, travers etc
My instructor always tells us to do circles, Serpentines and figure 8's for example, as well as transitions to get the hind legs working, and to relax your horse to get him more rounded :)