The same is true of collection. When a horse starts learning to collect itself for you, his weight shifts back, his shoulders elevate and he shortens is stride. So, I like to go at it from that direction. I find it is easier for me to teach a horse to use his hind quarters by teaching him to collect himself first.
When this has been accomplished, he knows how to start using his hind end, to elevate his shoulders and to not be as heavy on his front end. This knowledge and responsiveness will carry over when I start teaching him to lengthen his stride.
When I think he is ready, I will start out by sitting back and collecting him around the turn on a large arena (first at the trot and later at the canter), staying back (never leaning forward) and asking for more impulsion and a longer stride on the straight sides.
When he can do this, you usually start getting the difference in length of stride and not just the difference in speed only.