I start out by using a single pole and canter my horse up to it from one direction and break to trot over it a few steps out, picking the canter up (going the other direction this time, so opposite lead) as soon as I'm on the other side. Repeat this exercise coming from both ways, kind of like a figure eight .
This encourages your horse to swap leads over the pole while he already has his front feet off the ground. Remember to add the cue you want your horse to know as you do this (maybe outside leg inside rein). Keep practicing and be patient, it wont happen over night but if you're consistent, your horse will get the hang of it! :)