There are so many different ways!! Personally I'd use a combination of different exercises to improve the hindquater engagement. Firstly I'd make sure that my horse was relativley fit and sound - get the back, teeth and legs checked before you start. This way you know that there is no reason that  they can't engage the hindquaters and they won't be in any kind of pain. 

I would start off with some good solid hill work - lots of transitions up a hill. Walk - Trot, Trot - Walk, Trot - Canter and so on. The more comfortable you horse gets with this the more advanced you can make your transitions, Collection - Medium - Extension and back. This will start to get your horse using his bum. 

Then I would do the same down hill, but making sure that the horse is very balanced. This will help him to get his hind legs underneath him. But ALWAYS remember to do little bits at a time as this is really hard work for them to have to learn. 

In the school there are loads of little bits you can do to help. Like collecting the trot work down the short side and extending down the long side, making sure that you keep the balance and the rythm through the transition - only push as much as you can so you can get him back and underneath you. The better he gets the more elaberate you can make the transitions - Big extension - really small collection. Don't worry if this won't happen right at the beggining, it will take time and learning. Make sure that you give lots of breaks and do something different every so often. 

The other thing you can work on is poles! Start off with them on the ground and always an odd number set to your horses normal stride length. Once he's got used to this you can shorten them slightly so he has to sit back on his hocks. If you still find he isn't using his back end you can raise one side of each pole so he really has to pick up his legs and use them!! 

Hope this helps and let us know how it goes!! :D x