How do you prevent burn out from happening?Well, in a word: relax, man.
As tempting as it may be when you're really into your art, I would advise not going to every single class and every clinic, like I did. Pace yourself, champ. Budo is, as they say, a marathon not a sprint.
There are people who exist as living exceptions to that, of course; there will always be those rare few who seem destined to do this stuff as their primary purpose in life, but you need to brutally honest with yourself whether or not that's you. And for many, that may take years to figure out.
Avoid major life events
Okay, I'm kidding. Obviously, you can't avoid major life events, expected or not. So to that end, you might as well just accept that they will come and they will derail your training for a bit, and be okay with that. Enjoy the break.
That being said, there are also many different kinds of training. There are still plenty of opportunities to keep your head in the game, even when you can't physically make it to the dojo: books, DVDs, apps, blogs, podcasts, videos, conversations with fellow students, or even just going over the movements in your own mind.
Vary your interests
Even if budo is a major interest in your life, a big part of avoiding burn out is maintaining other passions outside of it. Even the samurai studied things like poetry, tea ceremonies, painting, flower arranging, etc. In other words, your yin could use some yang.
For me, I'm a graphic designer and illustrator by trade, and I derive a great deal of pleasure and satisfaction from that world. I also enjoy cooking, and even, on occasion, being with my kids =) Not only does focusing on one interest allow you to take a break from another, I think you'll be surprised at how stuff you learn and skills you gain from one interest will benefit another.
Routine vs chaos
In a nutshell, managing your training career over a lifetime is a delicate balancing act between knowing when to buckle down and focus with repetition, and knowing when to throw yourself a curveball. It's also part of being a good teacher, by the way.
All that being said, there are undoubtedly a whole host of reason why burn out occurs, all tailored to each individual. And just because I've dealt with it in the past and learned a few things doesn't mean I'll never face it again.
But it seems like that's all part of the journey. There's so much more to studying a martial art than just learning a handful of techniques, n'est-ce pas?