Set two poles on either side of the jump to guide him to go straight and if he or she thinks about refusing he or she will have to go over
I would give him lots of leg before the jump, so if he likes to run out towards the left squeeze him forward with your left leg.
halt him in front of the jump, don't let him go around it. If he goes around it, stop him and back him up so he is in front of it and make him at least walk over it
If I were you and my horse ducked out make them at least walk over the jump (unless your at a show) and even better get them to jump it from a stand still. DONT LET HIM/HER GET AWAY WITH IT!
Try doing some grid work/ place canter poles before the jump to give him confidence to go over in a rhythm. That way you don't have to worry about doing anything except to keep him straight - this is working with my little thoroughbred