The biggest advice I could give that I've learned from personal experience is that, in order to any kind of training method, you have to be open minded. I've gleaned so much over the years from parelli and clinton anderson and so many others, but I find that to study just one trainers tequniques doesn't fill in all the holes. Too many people fall so in love with a trainer's personal method that they come to believe that that trainer has all the answers. Don't fall into that pitfall. If one trainer in particular knew all the answers, then he/she would put all the other trainers out of business. Yet today we have so many trainers earning a decent paycheck even though they don't all believe the same things. Why? Because everybody has something to learn from, even if they don't go under the title of "natural horsemanship". Now there comes a point when you do have to use common sense. Some people make a living doing crappy or abusive training by preying on people who don't know any different. And that goes for the traditional and NH crowd alike. But that aside, I definately think that every good trainer has something to offer.
The second thing I've learned from experience is that before you use any training tequniques of any kind whatsoever, you should know why and how they work. Don't use some technique that was demonstrated who-knows-where that looked successful but you really don't quite understand. That's how alot of yahoo trainers everywhere give other trainers a bad name. They use half thought out ideals or techniques that ruin a horse and then label their style after someone else that never intended on their techniques being used that way. Even if you don't intend on training other peoples horses, you might do more harm than help to your own if you don't understand the techniques that you use. I highly recomend that anyone looking to train -anything- read the book "Don't shoot the dog" by Karon Pryor. It's not a book about dogs, but about the science of teaching and about how animals "and people" learn. Anyone who trains should understand the differences between positive and negative reinforcement and positve and negative punishment and how and when to use each appropriatley. Every training method known to man can be categorized by these labels. I also believe that anyone who trains horses should have a solid, basic understanding of horse psycology and behavior (Which, thanks to our Natural Horsemanship trainers of today, is much more readily available to the public! ^^)