Draw reins, like side reins are a great way to keep contact! They help keep your horse underneath them and rounded. The only downside is that they are easily misused. Make sure you know someone who can show you how to use them correctly or do some research on it. :)
Using your leg helps to push the horse into the bit. If he's not listening to leg or is dead to the leg use a spur to encourage him further. Working one side of the ring with an extended trot and then collected on the short side allows them to be free and then come back which helps with them pushing their hind end so they can stretch into the bit. Hope this helps:)
I would say use half halts before any bend,transition or just whenever you feel the horse needs it. This ensures the horse is connected with its whole body and the head should stay in settld in one position.
Hope this helps.
To help keep your horse in the collected position you can use side reins to help encourage the horse to place its head in the correct position or you could use a set of rings and just set them to stop the horses head from going too far up not to keep them down, or it sometimes just takes time and regular steady soft work. Another option would be to use a lunge roller and lunge your horse in side reins, starting rather loose and then slowly gathering them in to help the horse collect. This should all be done steadily because in order to achieve collection you will need to build up your horses back and neck muscles, this will happen as you start collection work. Hope this helps