What if each Ghast you killed earns you some kind of fear toward Ghasts, so after you kill at least 5 or more you get fear from ghasts. Then you tame it with ghast tears. And to ride it you can make a saddle out of nether warts. To control which way it goes is to have gunpowder in a pointed in a direction. Sorta like the pigs.
Edited by Deuster, 07 April 2013 - 04:53 PM.