There are alot of different things you can do.
I would try just limbering, working at getting long and low in the bit, softly and using the hindend, to be engaged.
You can try sidepassing, use a wall if you need to at first. First, face your horse toward the wall, use one rein to block the horse from going the way you don't want him too, and open the rein on the side you want him too. Apply pressure from the leg to move him over to one side, his front legs should cross as well as his back.
You can also try carrot stretches, which work very well, a video is provided from another user above.
I would just work on getting the horse soft and supple, depending on the age you could also try previcox, which works very well if your vet feels it is right for your horse. MSM is also great.
Hope this helps! Just be patient, it takes awhile for them to understand what you want ;)