Poles are just another method of teaching them. Works for some horses, doesn't work for others. It depends on the horse, some are 'predisposed' I guess you could say, to changing late behind, others will change clean from the get go.
A method I like is to first get the counter canter very strong - as someone above mentioned, if you do not have a strong, balanced counter canter before you start changes, you're going to run into big problems when you start NEEDING the horse to stay in counter canter. I'll put the horse into counter canter on a 20m circle, say on the left rein and cantering right lead, and ask from that, so a half halt in the CC, and ask for the change. Asking on the circle from counter canter aids balance so you're not 'throwing' the horse across to change leads, naturally changing leads back to the left lead is going to be more comfortable than counter cantering the circle, so most horses tend to get a clean change almost straight off.
But again, just depends on the horse, this method doesn't work for some, they get too clued into being able to change in counter canter and you have to start changing them from a true canter back to a true canter on the opposite lead.