Great article! I have a horse that is really hard to get into a canter. Definitely going to be working on haunches-in with him. I have taught my other gelding to do haunches-in and it really helps get the right lead. In teaching haunches-in, you also have to get them loosened up and soft throughout their whole body which I think helps all around.
Jim McDonald gave solid advice for smoothing the lope. What he neglected to share was that after all the preliminary work is smooth, the learning to smooth the lope part could perhaps add-up to over 100 hours of loping practice. I find that riders expect a rocking horse lope after just a few trys.