Transitions. Lots and lots of transitions. You can start fro the halt to walk, walk to halt. See if you can nudge your horse forward with the feeling of his hind end pushing first. If your horse gets heavy in front, stop, back up or rock him back, then nudge forward again. Repeat alot, and transition often. Only a few steps of walk before you halt. Each time you ask for him to walk forward, there is a lightness in your seat, but you maintain a soft even contact on the reins. Don't throw them away when you ask for forward. Be still in the saddle, don't tip forwards when you want him to walk forwards. Use a whip if you need to, to tap him up underneath you. Don't allow forwards until you feel him rock back. With each transition he will push more with his hind end, which is creating impulsion. He cannot push from behind if he is too down in front, too heavy in front, and too much on the forhand. That is why you either rock them back, or back them up. Even in the back up horses can be down in front. Raise your hands towards the sky and "bump" him up.
You can work your way from walk to trot, trot to walk.
Millions of transitions will create impulsion. Note that you need to maintain an even, soft contact with the reins so that your horse has a clear place to push into. If your rein contact is always varying, he will have a hard time finding where he is supposed to be. Just food for thought. :)
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.