I think it's somewhat delusional to think you can motivate or change people. If people are open and motivated to change, that's another story.

I think the best thing you can do is to know your audience. Who are you trying to help and why? Do they want your help? People are generally not receptive to unwanted advice. If you have clients that are seeking your help the best way is to know them as much as possible. Work with their personality and come up with creative solutions together.

You might want to read about Social Learning Theory and Self-Determination Theory.

a fascinating article I read recently:

http://youarenotsosmart.com/2010/07/07/extinction-burst/

it's more concept than application, but illuminating nonetheless.

My answer for your question #2 based on my real life experience:

The single most important factor in changing my lifestyle was greatly reducing (almost to the point of eliminating) time spent watching television. Without this giant time-suck, I was free to determine what behaviors I really wanted to nuture/develop without the influence of marketers, celebrities and fantasies. Television only provided me with "wishes": I wish I could cook like that! I wish I looked better naked! I wish I had time to exercise! I wish my flower garden was that nice! Without television those wishes in my head finally had to be acted upon to prevent boredom, and therefore behavior changes were a natural progression.

I now have 10 extra hours a week to cook, exercise, read, play with family/kids/pets, sleep etc. That is the equivalent of having an extra day in my week!

Here, Karen Pryor's book, "Don't Shoot The Dog" is one of the best books on the subject. Check it out: http://www.amazon.com/Dont-Shoot-Dog-Teaching-Training/dp/0553380397/ref=ntt_at_ep_dpt_1

In a nutshell, changing the antecedents to a behavior (instructions, environment, goals, etc.) and/or the consequences of a behavior (especially immediate feedback and reinforcement) can modify target behaviors.

There are all kinds of delicious subtleties, though. Are you seeking to increase or decrease a behavior? Is the behavior observable and measurable? Things that are considered reinforcement by the layperson (like employee of the month) are actually not contingent on behavior and therefore have no effect. And so on....

I teach learning and behavior at the university level, so if you have more questions I'd be glad to help.

Another book not yet mentioned is Bringing Out the Best in People by Aubrey Daniels. Enjoy!

Intrinsic rewards are crucial, not extrinsic. Meaning people need to find a reward inherent in the behavior itself, not an external reward (like money). For instance, research shows that when you pay kids to read books, while it does increase their reading, once the reward is removed, they actually read less than if they were never rewarded. After all, they are no longer getting the "reward" so why should they do it now? Instead of just naturally enjoying reading a book.

Probably explains why the biggest loser works so well up until the point they receive $, and then after that point, the weight comes back on.

I think when people eat the right foods, and get enough of them, they naturally feel better and WANT to be more active.

The lack of activity is due, I really honestly think, to people eating crap food and feeling like crap. Just take a peek at the soda cooler of any store, it'll tell the tale. I mean, does anybody think we have enough energy drinks yet?