1) Read the latest books on training and learning theory (not just dog-specific, but in general, and you can also learn a lot from the exotic animal training field).
2) Join APDT ( http://www.apdt.com
) - they are still probably the best resource for dog trainers; they also offer health, life and liability insurance for trainers. IAABC ( http://www.iaabc.org
) is growing,a great resource, and I've been really impressed with them so far.
3) Get as much hands-on experience as possible. Volunteer with a shelter or rescue. You can learn SO MUCH from rescue dogs. When I first started out, I offered services free or deeply discounted to people who adopted dogs from the rescues I was volunteering with. This is a great, non-pressure way to get experience teaching people.
4) Attend seminars. APDT is a good resource for this.
5) You don't have to become certified, but since the field seems to be learning towards a more structured state and/or licensing, certification may help you get ahead of the game. Also, for the purposes of letting your clients know you are a legitimate trainer, certification can be helpful. There are TONS of "certification programs" out there. The only ones I put stock in are the CPDT, IAABC certification, and the various behavior consultant/behaviorists orgs' certifications out there. Some of these require at the very least a master's degree. CPDT and IAABC require field-related knowledge and experience, only, no degrees.
6) And speaking of degrees, a psych degree is a great help.
Just some stuff I had to figure out the long and hard way.