First and foremost we need to be assured that there is no underlying medical reason why the horse is so heavy on the front, even though being heavy on the forehand is a VERY common occurrence in horses. Then we need to strengthen our OWN riding position in order to help our horse. Always stay nice and tall in the saddle, with you legs light on the sides, never grip with your thigh, use your calf. And make sure that YOU are properly balance in the saddle, before we ask your horse to re-balance itself.
Well, there are a several ways to encourage a horse, no matter what the discipline, to engage their hind quarters. The easiest? Transitions, transitions, transitions. Halt to trot, to walk to trot to lope to halt to back to on and on and on. Practicing transitions will force your horse to utilize it's hind end more, because you are constantly switching the gaits. Soon the horse will begin to anticipate your transpositions, and will therefore stay back on it's rump in preparation for your next move.
Also, circles, ahhhhhh you can never go wrong with a nice circle. Now if you are doing the circles properly, your horse will also be forced to engage his hind end. Be Careful not to mistake a bent in neck and nose as a properly bent horse. You need his body to be wrapping around your inside leg. This pic is a great example of that.