With Milo? We just kept canteirng him, and if he was on the wrong lead, we would go back to the trot and keep trying to go from trot to canter without picking up the wrong lead. Does that make sense? We just had to be consistant... we never really "tought" him. He used to be so bad that it was considered an amazing lesson if he picked up his right lead once... even if he bucked the person off, if he got the right lead, it was HALLAJUAH! Lol.
I teach side passing early in the training. I then use that to teach leads further on. If I want the left lead I use the right lead behind the girth to move the horse over to the left and at the same time encourage a lope. I do not trot when asking for a lead. Either a dead stop or a walk. As soon as I lay the right leg on the horse he moves to the left and I pull the right rein causing him to crab slightly to the right burying the right shoulder and exposing the left lead and with the asking for a lope he naturally breaks into that lead.