Character Motivation

Over on YAtopia is a good post on character motivation:

The problem with flimsy motivations is that they make the entire story unbelievable. Your characters don't have to do things exactly as the reader would do them - but they do have to act like a "normal person" would be expected to act. Without those believable motivations, the reader is unable to suspend disbelief. And when disbelief enters the mind, suddenly everything in the book starts to look sketchy.

So, how do you make sure your c...