What's working for me is just consistency.
When he gaits, he gets LOTS of praise and rewards (at first you might need someone on the ground to confirm when he is gaiting and when he's doing something else. I got mine with no experience in the gaited world, and it took me a while just to understand from the saddle what his feet were actually doing).
Once he realizes gait=good he'll give it to you more often, and then you can start bringing him back down to a walk when he trots or paces, and re-ask until he gaits instead. But he needs to know gait=good before he'll understand why you're re-asking.
If you aren't getting the gait at all, start with learning to control his feet with your legs at the walk - press your leg against his side as he steps forward with his hind leg. Then work on speeding that up, slowly over time, while keeping the 4-beat rhythm - until he finally breaks into the gait without going to a trot or pace.
Don't expect a gait for a long period of time for a while. Gaits use different muscles, so it takes time to build them up. Initially be glad for just a few steps, then work on increasing it once you can consistently get it when you ask.
There are some ways of breaking the pacing, too, which I'm not too familiar with, as so far I haven't had that problem with him (he does pace coming down from a canter, but I think that's a symptom of him cross-cantering, so I'm working to fix that first).
I highly recommend Gaits of Gold by Brenda Imus. The books gives some exercises for strengthening the muscles a horse needs to gait, and getting them used to lateral motion. It also has some discussion about how to stop the trot and the pace.
In my journey, I've had good luck asking questions in the Gaited Horses forum (under Breeds). There are a lot of knowlegeable gaited riders and trainers there.
Good luck. Just be patient and consistent. It will come.